Many of the things you write sound OK. But I wouldn't say that the other directions in the graph are mere strengths, they indicate the actual value (strength/magnitude and direction as well) of the electric and magnetic fields.
First note that technically the electric field is a vector and it is a field so it should have a vector (possibly zero at every point). Each vector has a head and a tail. You can think of the location of the tail as telling you the place where the field has a particular value. Then you can think if the difference between the head and the tail as telling you both the magnitude and the direction of the electric field at that point, in some specified unit system. So imagine a bunch of arrows all the same color the location of each is telling you where it is telling you the field and how the arrow points from there tells you the value.
Then draw the magnetic fields in a different color. And in both cases you can't draw a vector at every single point because it would just be too much to see them all.
For a plane wave traveling through vacuum there is a great deal of regularity.
If the wave travels in the x direction then the electric and magnetic fields all point in the y-z plane. And their values only depend on time and on x, so you can draw just one longitudinal line and it will tell you about all of the values at all the locations. And since the electric fields and magnetic fields all point in the y-z plane you can reimagine the y-z plane as like the independent axis of a graph.
And for a classical plane wave travelling through a vacuum your picture should have the electric and magnetic fields both be strong together at one plane of a fixed value in the longitudinal direction and then at a different plane corresponding to a different fixed value of the longitudinal direction. Strong together and weak together and then strong again but pointing in the opposite direction.
If so, you get a picture much like the first one you drew (though that is just for one the electric and magnetic fields, a wave has both). That is all fine for a classical wave travelling through a vacuum.
For your other questions I think you should look at existing questions about the different topics.
From a quantum perspective it is much more complicated. From a quantum view there isn't an electric or a magnetic field, there is a photon field. When you have a large number of photons all in phase with each other it can look or act like an electromagnetic wave, but it is still different, and if you have a small number or they aren't in phase then it just truly different.
The electric and magnetic fields classically are stand-ins for saying how charges interact (though they do have their own energy and momentum, pressure and stress, etc.) And in quantum mechanics different interactions are possible and so the photon field is a stand in for saying how those different interactions go.
Since in quantum mechanics the charges don't have a location and momentum saying that their momentum changes based on the field where the charge is located just isn't going to be possible because you have none of those things. And if you can't verify the field's values at any location it becomes difficult to say it is there in that way.
We make the objects we need to predict results, for quantum mechanics we need different objects because we are predicting different interactions. Only in some limits (that only hold sometimes) do we expect quantum mechanical effects to start to approximately look like classical effects.
As for a transverse spacing in a Faraday cage stopping a wave that varies only in the longitudinal direction remember that you were drawing only the simplest case, the case of a plane wave.
In a plane wave the entire plane parallel to the y-z plane has the exact same electromagnetic field. This very simple (to describe) solution allows the wave to travel entirely in the x direction, over time each part of the wave simply slides over, the new value at distance $x=x_0+\Delta x$ over is the old value at $x_0$ a time $\Delta x/c$ earlier.
But no real wave is ever that perfect. They might expand in a spherical front instead of a plane front. They might have a central region where it is strong and it gets weaker farther out (like a Gaussian beam) some people even make beams where the energy and momentum travels along a cone so there is a central strong region but if you place a small object there the region beyond it still gets light because the light from farther out came from a base of the cone that started out farther from the central region.
Sort of like if you had some racers all lined up for a race but instead if running in their own tracks they ran towards the same point that is in front on the center runner. The center runner gets there first, but if parts of the center of the track are damaged the runners that left later will eventually close in and it won't matter that that the runners originally closer to the center got affected.
So a real wave has a more complicated wave front. And in fact if you are trying to avoid the wires of the Faraday cage you need your beam to be focused to not extend out too far in the direction transverse to the direction of propagation. And this isn't restricting the amplitude, this is saying that the actual wave shouldn't have field values spread out in the yz plane, it should be focused to a small portion of the yz plane.
Imagine you shot narrow beams all towards the same hole in the Faraday cage, you need very precision aiming each point has to has "its x" aimed very precisely to get in that same hole.
In general if you aim your initial propagation directions with accuracy on the order of the wavelength then you gave changed the wave.
Thus is because the direction of propagation isn't a magic thing you can freely associate with a point. It depends on how the electromagnetic field varied in space. When you field didn't vary in the y and z directions then the field travelled in the x direction. Now you are trying to get each region to aim slightly differently so you try to adjust the electric and magnetic field here and there to have each part aim just right.
Waves tend to spread, so if you aim some at that one whole, lots will be lost in other directions and what aimed well for one whole is nor going to aim for the other holes, so the Faraday cage succeeds at blocking almost all fields coming at it assuming the cage is big enough to have many many holes.
I think many of these question can be (and have been) asked as separate questions.
Typically we describe the electromagnetic field using the electromagnetic four-potential, which is represented as $\mathbf{A}$. From the four potential we derive the electric and magnetic fields using:
$$ \mathbf{E} = -\nabla \phi - \frac{\partial\mathbf{A}}{dt} $$
$$ \mathbf{B} = \nabla\times \mathbf{A} $$
where $\phi$ is the electric potential. It is very important to appreciate that the values we get for $\mathbf{E}$ and $\mathbf{B}$ are coordinate dependant, by which I mean that observers in different frames will measure different values for $\mathbf{E}$ and $\mathbf{B}$. You can see this very easily. Suppose in my frame I have a charge at rest, in which case that charge generates a static electric field and no magnetic field. If you are moving with respect to me then you observe a moving charge, i.e. a current, and currents generate magnetic fields.
So if you are considering an all encompassing field this would have to be the four potential. But the four-potential is not a physical object. It is a function of spacetime. If you feed a spacetime point into the function it will return a vector that describes the electric and magnetic potentials at that point.
In principle the four-potential is a function of the position and velocity of every charge present anywhere in the universe. In practice we usually find we can ignore distant charges so the four potential is a function of only finitely many charges:
$$ \mathbf{A} = f_A(q_0, v_0, q_1, v_1, ... q_n, v_n) $$
But the function $f_A$ can be broken up into a sum of separate functions for each charge:
$$ \mathbf{A} = f_0(q_0, v_0) + f_1(q_1, v_1) + ... + f_n(q_n, v_n) $$
and we could write this as:
$$ \mathbf{A} = \mathbf{A}_0 + \mathbf{A}_1 + ... + \mathbf{A}_n $$
where you can regard $\mathbf{A}_0$ as the four-potential of charge 0 and so on.
Now back to your question:
At every point in spacetime there is just one value for $\mathbf{A}$. The four potential cannot simultaneously have more than one value. However we can write that value as a sum of the four-potential of every charge.
So there is a sense in which there is a single electromagnetic four-potential, but there is also a sense in which there are lots of individual electromagnetic four-potentials that add up to give a single value at every point.
I think which of these interpretations you prefer is philosophy rather than physics. My view is that there is a single four-potential and it can be mathematically decomposed into individual components.
Best Answer
1) First let us separate colour perception from frequency. Individual frequencies have a color correspondence but the colour the human eye perceives is another story.
2) White light, such as sunlight, is composed of many frequencies.
When the impinging wave hits a wall it can be a) reflected b) absorbed c) scattered incoherently
In order for the light waves to cancel out each other or double each other the photons have to be, within the uncertainty principle, superimposed in time and space. Sometimes it happens, but the probability is small. That is one of the reasons why a reflected beam can never have the same strength as its original beam. If the frequency is the same the probability will be higher than if the frequencies come from a random palette.
This superposition can be achieved with lasers, where there is control of frequency and the beam is coherent, i.e. the phases are preserved upon reflection. A hologram is an example of superposition of same-frequency light to create a three dimensional shape by peaks and dips.
Edit: From a disappeared question the following comment is worth adding: