..... Question #1: Is an eighteen gauge nailer what I want? I will be
building a bookcase/desk out of ¾” plywood (to be painted) and I'll be
using the nailer to attach trim around the top, and to put on the
backs of the bookcases (½” plywood).

For trim, you'd be far better off with a 23-gauge pin nailer. I made
the mistake at first of getting one that drove short pins and had to
get a Grex that took 1 3/8" pins. I've also used it to attach face
frames. (Of course you can use shorter pins with a larger capacity pin
nailer.)


Question #2: Some nailers also drive staples. What are staples good
for? Upholstery? Insulation?

I've also seen staplers used on plywood casework that is going to be
painted.