1. I'd say that if you change both aging settings to .800 and leave the developmental settings as is, you'll be satisfied with the results. However, injuries have impacts on ratings, so take that into account (almost everyone would suggest setting injury frequency to 'low'.)
2. Obviously, there are so many different ways to play, and they all have their advocates and thus, they are popular. Historical leagues are very popular, though not to me, but I have heard that OOTP is mediocre at doing so. My favorite way of playing is playing GM mode, allowing my manager to makes up the lineups (with a few adjustments from me here and there), while I make signings and trades, and see how it all plays out. Two of the most exciting things in OOTP to me are seeing that risky draft pick pan out, and rebuilding a perennial doormat into a powerhouse.
3. Unfortunately, yes, although I think (I'm not sure as I don't like playing with real players) that there may be a league file that has the Nippon Baseball League built to at least somewhat good accuracy.
4. I am not averse in this category, but I will see what I can do to help. There is an option called recalc, which keeps players' numbers pretty accurate to real life. If you play with recalc off, Babe Ruth could end up being a 100 home run hitter while Fred McGriff could hit 600, it all becomes random. Also, there are other features, such as checking a box that force retires players in the actual year that they retired. That's about as much help as I can give.
5. That entirely depends on what you are intending to do. One great mod is
OOTPX companion, but there are really so many great ones out there. However, some are geared towards online leagues so those won't work for your game unless you run an online league. The creators of the mods usually describe how to input them.
6. Yes, you definitely do. Some common house rules are a limit on trades per year, as well as the number of players that you're giving away versus what you were getting (you can load up 7 or 8 garbage players to a team for a super prospect, so most people limit themselves). There are plenty of others but those are the only ones I really use.
Hope I helped!