Not sure I can answer all of your questions, but I'll give some of them a shot:
1. Steam is a way that some people buy games, but you can just buy straight from OOTP if you want to and it will install like any other game/program.
2. There are no 3D models of the actual minor league parks in the game, but you can play out the games in a generic 3D park. The
mods forum might have some real 3D models. I put together a bunch of information regarding park factors/pictures/etc but not actual models.
3. I personally mostly play with the get fired button turned off, so I can't help too much here. From some of the people I've watched, they can labor for a little while before getting fired. It might change based on your owner's personality.
4. Anything is possible! Many people require their own "house rules" as to not take too much advantage of the computer. The game does do a very good job, but sometimes there are ways to trick it into thinking that your extra no-good prospect will be enough to give you their 1st round pick. You may have to give up long-term success to do a good job in the next year or two of the league, though so you always have to figure out just what you want to do.
5. I've been trying to compile some resources and put them in the link in my signature, which you can check out.
Oh and one more thing: the forum search seems to not give the greatest results. Sometimes you can get a better result in the forum by searching using your favorite search provider using site:forums.ootpdevelopments.com *search term*