1. Can I generate teams with no players, or will the 400 fictional players be generated anyway?
No, and yes.
2. If the player generation is automatic, can I release all players to free agency and delete them? (It seems I can do this in Commissioner mode from the Free Agents Subpage -- is that correct?)
Yes and yes.
3. Though it would be tedious, could I import 400 historical players (25 per team), regardless of what era they played? (It seems I can also do this in Commissioner mode from the Free Agents Subpage -- is that correct?)
Yes and yes.
4. If I select the “peak seasons” setting, at what age will they import?
Whatever year you select.
5. Can I set each one to be age 21?
I suppose. Don't see why not.
6. Would they be imported directly onto the team of my choice, or to the free agents pool?
Pool I believe.
7. After the short season, will I be able to have all 16 teams play off?
Doubt it.
8. If not, if I divide the sub-leagues into divisions, am I limited to 4 teams per league in playoffs?
As far as I know.
9. After the playoffs, I want to release a few players per team, then import historical replacements who would be age 21 with ratings based on peak seasons. Doable?
Yes.
10. During the following season, would the players age as though they had played 154 games or ... what could I expect?
Season = year, essentially.
11. If injuries are off, do position players still get tired?
Yes.
12. Is spring training mandatory?
No.
13. Can weather effects be turned off?
What weather effects?
14. Can ballpark effects be turned off?
They can be neutralized.