I suggest the following lecture notes of Bruhat: 

www.math.tifr.res.in/~publ/ln/tifr14.pdf

Chapter 3 & 4 should answer most of your questions.

For example, there are statements like this :


Proposition 1(pg.19). To every analytic representation h : G −→ G′ there
corresponds a map dh : U(G) → U(G′) which is a representation of
algebras such that ( f ◦ h) = (dh() f ) ◦ h.

Corollary (pg.36). Let G and H be two Lie groups having g and J as their Lie
algebras. If G is connected and simply connected, to every representation
π of g in J, there corresponds one and only one representation f
of G → H such that d f = π.

Both give you an isomorphism between the categories. Equivalence would be a weaker statement.

Since you are interested in reductive groups only, the categories of representations are both semisimple, i.e., reps decompose into irreducible ones.

Thus, also your second and third question can be answered affirmative.