I can't answer all your questions but some.
1. Legacy games are basically emulated MKVI games that run on MKVII. Aristocrat eventually released native MKVII games on most popular titles.
2. Most setchips will allow you to choose earlier versions of the setchip (usually you choose from a few options) to match whatever game needs to be installed. 7.02.50 is too new for 3.05.0, but it may include 7.00.xx versions on your setchip. You have to look at your available options.
3. No generic variation that works, you have to select the game specific game type. If you don't have a PAR file, then it just becomes a guessing game. You can also post a question on the forum, lots of people here can probably provide and answer that will work, most newer MKVII titles (not legacy) are BaseX Multigame. The other wildcards are whether or not they support multi-denom and the game variation supported (I usually guess either 99 or 1).
4. Not sure...
5. This is the same as question 3, you need the PAR file or start guessing.
6. Not possible to get info without par sheet, ask in forum, most of the times, you will get something that works. Unfortunately PAR files are only made available to casino techs, so unless you know someone who is a casino tech and is willing to share them, you have to get lucky someone will the PAR file for the game you need and is willing to share it.