Your first question is about two objects becoming isomorphic after quantization, and you're asking "Why?"
Here, the relevant quantum object is the spin representation of mathfrakgmathfrakg, which is a representation of mathfrakgltimesmathitCliff(mathfrakg),
where mathitCliff(mathfrakg) is the Clifford algebra of (the underlying vector space of) mathfrakg, with respect to some invariant inner product; a mathbbZ/2-graded algebra.
Let S be the unique (up to grading reversal) irreducible mathbbZ/2-graded representation of mathitCliff(mathfrakg). It has a graded-commuting action of
C:=begin{cases}mathbb C&text{ if }quad dim(mathfrak g) text{ is even} \ mathit{Cliff}(1)&text{ if }quad dim(mathfrak g) text{ is odd.} end{cases}
Let V be the category of modules of the above algebra, so that SinV.
To make things a bit more canonical, one can use the graded Morita equivalence between C and mathitCliff(mathfrakh) to identify V with the category of mathitCliff(mathfrakh)-modules.
Let alpha denote the adjoint action of G on mathitCliff(mathfrakg). For any element ginG, we can pre-compose the action of mathitCliff(mathfrakg) on S by alphag to get a new, isomorphic mathitCliff(mathfrakg)-module in V [here, I'm using that G is connected].
The map that sends g to such an isomorphism is unique up to scalar, and so we get a projective representation of G on S.
If G is simply connected, this lifts to an honest action of G on S, and so we get an action of GltimesmathitCliff(mathfrakg) on the object SinV.
All in all, (the underlying vector space of) S has actions of G, of mathitCliff(mathfrakg), and of mathitCliff(mathfrakh).
Now, we can "cancel" two mathitCliff(mathfrakh) actions to get a vector space with actions of G and of mathitCliff(mathfrakgominush).
That's the irreducible G-rep with highest weight rho.
This vector space has two descriptions:
(1) The irreducible G-rep with highest weight rho.
(2) The irreducible mathitCliff(mathfrakgominush)-module.
Note however that this operation of "canceling" the two mathitCliff(mathfrakh) actions is a bit unnatural.
For example, the vector space (1) is purely even, while (2) is genuinely mathbbZ/2-graded. You probably see that same weirdness on the classical side of the problem when you try to identify the flag variety with a product of mathbbP1s.
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