1.
Like Veld, plus you can marry individuals by editing your save game file. (See
here.)
2.
Like Veld, plus playing as a nonXian, to create new titles you need to edit the liege to be a Xian (ibid.) then change it back (ibid.)
Playing as a Xian, to gain the Muslim- or pagan-specific titles you need to force-vassalize the current holder, then revoke his title. DOW as necessary. This will not work for king-level titles. For those, you will need to destroy the current holder and then modify the save game file to give it to your liege.
3.
"Get under?"
Do you mean become a direct vassal? You just need to be independent of any other lord, have a small amount of prestige (25?), and then offer your services directly to the emperor.
If you mean become the immediate heir, it depends on his inheritance law (which you can modify in the saved game.) When they are elective, you just need to be the strongest vassal (see
here.) When they are semisalic, you just need to be married to his sister or daughter and kill off his male children. When they are salic, you need to edit the saved game to change that because otherwise you're SOL.
4.
There are no world-bestriding emperors, just obnoxious kings. And even the Greek imperial title basileus is just the old Greek word for king.
5.
Why would you need to? The ingame surname isn't exactly a nuclear family. But no, there's no way to enforce the church's increasingly strict prohibitions on marriage within families. Which isn't a deal-breaker because everyone routinely ignored those prohibitions anyway.
6.
You want a hex code editor. Google around.