1) Culture settings have bonuses and/or penalties to such things as space combat, maintenance rate, and happiness. Load SE4, and "add new empire" to see the details.
1b) Demeanor, I believe is just a descriptive thing, but I don't know if SE4 will complain if you enter a new one.
2) You are editing AI_general.txt? There are three sets of entries in there, the first is for 2000 point games, the second for 3000 point games, and the Last for 5000 point games.
See SE4's "Add new empire" to make sure the items you choose will add up to exactly that price.
3) Right. There is a viewer, possibly on the CD, that will display the whole shipset, and has a toggle to change the transparent parts from black to hot pink and back, so you can be sure it looks right.
4) No. All races start with the techs defined by techareas.txt. The specific techs given also depend on whether the game is a low, medium, or high tech start.
5) That file is critical. For example: If your shipdesigns.txt call for ships with Antimatter Torpedoes, you darn well better research that tech area early!