The answer to all questions is No. In fact, even the right reaction to the first sentence - that the Planck scale is a "discrete measure" - is No.
The Planck length is a particular value of distance which is as important as $2\pi$ times the distance or any other multiple. The fact that we can speak about the Planck scale doesn't mean that the distance becomes discrete in any way. We may also talk about the radius of the Earth which doesn't mean that all distances have to be its multiples.
In quantum gravity, geometry with the usual rules doesn't work if the (proper) distances are thought of as being shorter than the Planck scale. But this invalidity of classical geometry doesn't mean that anything about the geometry has to become discrete (although it's a favorite meme promoted by popular books). There are lots of other effects that make the sharp, point-based geometry we know invalid - and indeed, we know that in the real world, the geometry collapses near the Planck scale because of other reasons than discreteness.
Quantum mechanics got its name because according to its rules, some quantities such as energy of bound states or the angular momentum can only take "quantized" or discrete values (eigenvalues). But despite the name, that doesn't mean that all observables in quantum mechanics have to possess a discrete spectrum. Do positions or distances possess a discrete spectrum?
The proposition that distances or durations become discrete near the Planck scale is a scientific hypothesis and it is one that may be - and, in fact, has been - experimentally falsified. For example, these discrete theories inevitably predict that the time needed for photons to get from very distant places of the Universe to the Earth will measurably depend on the photons' energy.
The Fermi satellite has showed that the delay is zero within dozens of milliseconds
http://motls.blogspot.com/2009/08/fermi-kills-all-lorentz-violating.html
which proves that the violations of the Lorentz symmetry (special relativity) of the magnitude that one would inevitably get from the violations of the continuity of spacetime have to be much smaller than what a generic discrete theory predicts.
In fact, the argument used by the Fermi satellite only employs the most straightforward way to impose upper bounds on the Lorentz violation. Using the so-called birefringence,
http://arxiv.org/abs/1102.2784
one may improve the bounds by 14 orders of magnitude! This safely kills any imaginable theory that violates the Lorentz symmetry - or even continuity of the spacetime - at the Planck scale. In some sense, the birefringence method applied to gamma ray bursts allows one to "see" the continuity of spacetime at distances that are 14 orders of magnitude shorter than the Planck length.
It doesn't mean that all physics at those "distances" works just like in large flat space. It doesn't. But it surely does mean that some physics - such as the existence of photons with arbitrarily short wavelengths - has to work just like it does at long distances. And it safely rules out all hypotheses that the spacetime may be built out of discrete, LEGO-like or any qualitatively similar building blocks.
The strings do not attach to the space-time manifold, they move around on it as a background. Option 1 is not right.
Option 2 is more like it, except that you are assuming that string theory as it is formulated in the string way builds up space-time from something more fundamental. This is not exactly true in the Polyakov formulation or in any of the string formulations (even string field theory). The string theory doesn't tell you how to build space-time from scratch, it is only designed to complete the positivist program of physics. It answers the question "if I throw a finite number of objects together at any given energy and momentum, what comes out?" This doesn't include every question of physics, since we can ask what happens to the universe as a whole, or ask what happens in when there are infinitely many particles around constantly scattering, but it's close enough for practical purposes, in that the answer to this question informs you of the right way to make a theory of everything too, but it requires further insight. The 1980s string theory formulations are essnetially incomplete in a greater way than the more modern formulations.
The only thing 1980s string theory really answers (within the domain of validity of perturbation theory, which unfortunately doesn't include strong gravity, like neutral black hole formation and evaporation) is what happens in a spacetime that is already asymptotically given to you, when you add a few perturbing strings coming in from infinity. It then tells you how these extra strings scatter, that is what comes out. The result is by doing the string perturbation theory on the background, and it is completely specified within string perturbation theory by the theory itself.
Option 3 is sort of the right qualitative picture, but I imagine you mean it as strings interacting with a quantum gravity field which is different from the strings, strings that deform space and then move in the deformed space. This is not correct, because the deformation is part of the string theory itself, the string excitations themselves include deformations of space-time.
This is the main point: if you start with the Polyakov action on a given background
$$ S = \int g_{\mu\nu} \partial_\alpha X^\mu \partial_\beta X^\nu h^{\alpha\beta} \sqrt{h} $$
Then you change the background infinitesimally, $g\rightarrow g+\delta g$, this has the effect of adding an infinitesimal perturbation to the action:
$$ \delta S = \int \delta g \partial X \partial X $$
with the obvious contractions. When you expand this out to lowest order, you see that the change in background is given by a superposition of insertion of vertex operators on the worldsheet at different propagation positions, and these insertions in the path-integral have the form
$$ \partial X^\nu \partial X^\mu$$
These vertex operators are space-time symmetric tensors, and these are the ones that create an on-shell graviton (when you smear them properly to put them on shell). So the changing background can be achieved in two identical ways in string theory:
- You can change the background metric explicitly
- You can keep the original background, and add a coherent superposition of gravitons as incoming states to the scattering which reproduce the infinitesimal change in background.
The fact that any operator deforming the world-sheet shows up as an on-shell particle in the theory, this is the operator state correspondence in string theory, tells you that every deformation of the background that can be long-range and slow deformation shows up as an allowed massless on-shell particle, which can coherently superpose to make this slow background change. Further, if you just do an infinitesimal coordinate transformation, the abstract path-integral for the string is unchanged, so these graviton vertex operators have to have the property that coordinate gravitons don't scatter, they don't exist as on-shell particles.
The reason this isn't quite "bulding space-time out of strings" is because the analysis is for infinitesimal deformations, it tells you how a change in background shows up perturbatively in terms of extra gravitons on that background. It doesn't tell you how the finite metric in space-time was built up out of a coherent condensation of strings. The question itself makes no sense within this formulation, because it is not fully self-consistent, it's only an S-matrix perturbative expansion. This is why the insights of the 1990s were so important.
But this is the way string theory includes the coordinate invariance of General Relativity. It is covered in detail in chapter 2 of Green Schwarz and Witten. The Ward identity was discovered by Yoneya, followed closely by Scherk and Schwarz.
The point is that the graviton is a string mode, a perturbation of the background is equivalent to a coherent superposition of gravitons, and graviton exchange in the theory includes the gravitational force you expect without adding anything by hand (you can't--- the theory doesn't admit any external deformations, since the world-sheet operator algebra determines the spectrum of the theory).
In the new formulations, AdS/CFT and matrix theory and related ideas, you can build up string theory spacetimes from various limits in such a way that you don't depend on perturbation theory, rather you depend on the asymptotic background being fixed during the process (so if it starts out flat, it stays mostly flat, if it starts out AdS, it stays AdS). This allows you to get a complete answer to the question of scattering on certain fixed backgrounds, and get different pictures of the same string-theory spectrum in terms of superficially completely unrelated gauge fields or matrix-models.
But you asked in the Polyakov string picture, and this is only consistent for small deformations away from a fixed background that satisfies the string equations of motion for the classical background.
Best Answer
Let me try to address your questions, even though just the first one seems quite heavy in itself.
Spacetime Discreteness: let me give you links to references that are relevant to your questions, than i'll make some general comments. An Introduction to Spin Foam Models of Quantum Gravity and BF Theory; Spacetime in String Theory; The quantum structure of spacetime at the Planck scale and quantum fields; Meaning of Noncommutative Geometry and the Planck-Scale Quantum Group; Causal Sets: Discrete Gravity (Notes for the Valdivia Summer School); On the Origins of Twistor Theory — this should get you going. As for string theory and spacetime discreteness, let me say that, in a crude way, the $\alpha$ that appear in the Action in this link Superstring theory, called 'string tension', is basically what 'measures' this.
Other approaches not listed: i think your list is fairly complete. But, you didn't list Loop quantum gravity — maybe you were thinking of it, or maybe it fits in one of your named categories: i just thought i'd make it explicit.
GR discreteness: to me, this is a subtler question, in the sense that once you have discretized spacetime (for one reason or another), you should not expect that the other [geometric] structures remain 'continuous' — in fact, there's a whole branch of research dealing with 'quantum groups' and 'discretized' (or 'latticized': think computer simulations) theories. The point being that if you discretized all of your ingredientes, you still maintain a certain relation among them (e.g., discrete gauge symmetry, or $q$-gauge symmetry). The bottom line is that you can perfectly define a theory where all ingredients are properly 'discretized', and so it maintains its relevant features (recovering the continuum theory in some limit). As a side note, it's worth seeing that it's possible to discretize theories at the level of differential forms, à la Discrete Differential Forms, Gauge Theories, and Regge Calculus (PDF) (and similar constructions by several other folks). In this sense, many of the relevant properties are kept even after discretization (quite robust method).
I hope this can get this discussion started.