These are mainly my experiences...

1) I'm going to be using ABS for the DWV. What are the pros and cons
of a solid ABS toilet flange versus one with a metal flange?

I use a metal flange that can be adjusted by spinning the outer metal
part. Let's me slap everything together and adjust for proper
orientation later. More importantly would be the wax seal I'd think.

2) Should the top of the toilet flange be flush with the finish floor,
or 1/4" higher? I've heard both.

I've heard both as well. I go with flush. You can always stack wax
seals if there's a problem.

3) Does the use of a sanitary cross to connect two toilets to a soil
stack force the soil stack to be 4" in diameter? If so, should I
just make the toilet drains 4"?

Don't today's low-volume toilets require 3"?

4) Every diagram I have seen shows a closet bend right beneath the
toilet flange. Can it be lower? I'd like to drop below some 2x10
floor joists, as I have plenty of altitude and room in the crawl
space.

Dunno. I've often wondered if this is by design of the toilet or
house.

5) For that matter, is using a closet bend a requirement, or is it
just a convenience? I can imagine using a 45 degree bend and a Y
connection.

Again, dunno. 2x45 would be more gradual but would introduce more
edges for 'crap' to get stuck on (how much would be caught in the
lifetime of a house is another question). I'd guess the answer to #4
will help here.

Like I said, only my experiences.
Darryl.