Borel's lengthy 1953 Annals paper is essentially his 1952 Paris thesis. It was
followed by work of Bott, Samelson, Kostant, and others, which eventually answers your
side question affirmatively. For a readable modern account in the setting
of complex algebraic groups rather than compact groups, try to locate a copy of the lecture notes: MR649068 (83h:14045) 14M15 (14D25 20F38 57N99 57T15)
Hiller, Howard,
Geometry of Coxeter groups.
Research Notes in Mathematics, 54.
Pitman (Advanced Publishing Program), Boston, Mass.-London, 1982. iv+213 pp.
ISBN 0-273-08517-4. (This was based on his 1980 course at Yale. Eventually
he left mathematics to work for Citibank.) The identification of the cohomology ring with the coinvariant algebra of the Weyl group has continued to be important for algebraic and geometric
questions, for instance in the work of Beilinson-Ginzburg-Soergel. While Hiller's notes are not entirely self-contained, they are helpfully written. (But note that his short treatment of Coxeter groups has a major logical gap.)
ADDED: In Hiller's notes, Chapter III (Geometry of Grassmannians) is most
relevant. For connections with Lie algebra cohomology, the classical paper
is: MR0142697 (26 #266) 22.60 (17.30)
Kostant, Bertram,
Lie algebra cohomology and generalized Schubert cells.
Ann. of Math. (2) 77 1963 72–144. Nothing in this rich circle of ideas can be made
quick and easy; a lot depends on what you already know.
P.S. Keep in mind that Hiller tends to give explicit details just for the
general linear group and grassmannians, but he also points out how the main
results work in general, with references. I don't know a more modern textbook
reference for this relatively old work. But the intuitive connection between
the Borel picture and the Bott/Kostant cohomology picture is roughly this: The
Lie subalgebra spanned by negative root vectors plays the role of tangent space
to the flag manifold/variety. In the Lie algebra cohomology approach you get an explicit graded picture for each degree in terms of number of elements in
the Weyl group of a fixed length, whereas the Borel description in terms of Weyl group coinvariants makes the
algebra structure of cohomology more transparent. (What I
don't know is whether a simpler proof of Borel's theorem can be derived using Lie algebra cohomology.)
Concerning the relationship between $K/T$ and $G/B$, this goes back to the
work around 1950 on topology of Lie groups (Iwasawa, Bott, Samelson): all the
topology of a connected, simply connected Lie group comes from a maximal compact subgroup. So the two versions of the flag manifold are homeomorphic.
In later times, emphasis has often shifted to treating $G$ as a complex algebraic group, so that $G/B$ is a projective variety. For me the literature is hard to compactify.
One more reference, which treats the Borel theorem in a semi-expository style: MR1365844 (96j:57051) 57T10
Reeder, Mark (1-OK),
On the cohomology of compact Lie groups.
Enseign. Math. (2) 41 (1995), no. 3-4, 181–200. There is some online access here.
$G/B$ is most naturally a multi-projective variety, embedding in the product of projectivizations of fundamental representations: $\prod \mathbb{P}
(R_{\omega_i})$. So there is a multi-homogeneous coordinated ring on $G/B$. You mentioned that this ring is $\bigoplus_{\lambda\in P_+}$ $R_\lambda$. This is correct, and the grading is also apparent. It's given by the weight lattice. (To be more canonical, you should take the duals of every highest weight representation, but what you've written down is isomorphic to that.)
So all that remains is giving the multiplication law on $\bigoplus_{\lambda\in P_+}$ $R_\lambda$. You need to specify maps $R_\lambda \otimes R_\mu \mapsto R_{\lambda+\mu}$. There's a natural candidate: If you decompose $R_\lambda \otimes R_\mu$ into a direct sum of irreducible representations, $R_{\lambda+\mu}$ will appear exactly once. The multiplication law is simply projection onto this factor.
@Shizuo: For $sl_n$, the situation is more explicit. The flag variety here is the set of flags $0 = V_0 \subset V_1 \subset \cdots \subset V_{n-1} \subset V_n = \mathbb{C}^n$ with $\dim V_i = i$. So the flag variety is a closed subvariety of the product of Grassmannians $Gr(1,n) \times \cdots \times Gr(n,n) $. Each of these Grassmannians have a explicitly Plücker embedding into the projectivization of the exterior power of $\mathbb{C}^n$. In particular, the homogeneous ideal is explicitly given by the Plücker relations. So the multi-homogeneous coordinate ring of $Gr(1,n) \times \cdots \times Gr(n,n) $ is just the tensor product of the known homogeneous coordinate rings.
Finally, to get the multi-homogeneous coordinate ring for the flag variety, we need to specify an incidence locus inside $Gr(1,n) \times \cdots \times Gr(n,n) $. Namely, we need to specify those tuples of subspaces that form a flag. But this is easy: it corresponds to certain wedge products being zero. Just impose those additional relations, i.e. mod out by the corresponding multi-homogeneous ideal. Now you should have an explicit, albeit fairly long description of the homogeneous coordinate ring. The above answer for $SL_3$ looks like a special case of this construction.
Best Answer
Let $\mathfrak{g}$ be the Lie algebra of the group $G$. You might consider reading about the Springer resolution $$\mu:T^*(G/B)\rightarrow\mathcal{N},$$ where $\mathcal{N}$ is the nilpotent cone of $\mathfrak{g}$. The fibres of this map are isomorphic over individual adjoint orbits of $G$. These are called Springer fibres. For each such fibre, there is a representation of the Weyl group on its Borel-Moore homology (or dually, its cohomology). The fibre above $0$ is the zero-section of $T^*(G/B)$, giving us a representation of $W$ on $H^*(G/B)$.
A reference would be Representation Theory and Complex Geometry, by Chriss and Ginzburg.