From there, they've definitely established some sort of theme that the new holder should die before becoming immortal.
No they haven't. They've shown that C.C was covered in blood after the nun killed herself. We have no idea whether she would have died without a code, or if this somehow activated it, or even what the hell happened. Did the nun give her the code, then attack her, then kill herself, or what? Why would she do that? Maybe C.C got bored of immortality after um about 2 seconds and tried suicide? That whole scene makes no sense.
Charles shooting himself is just [tornado fang]ing with Lelouch, 'cause that's what he does. Note the complete lack of the typical Geass effects or red eyes. I mean, even if you do assume he needed to get killed to activate his code, he still did it in the way that would freak Lelouch out most rather than just shooting himself or whatever in private, so it's not like it's out of character.
You can assume codes activate as soon as they're given, and the show makes just as much sense as with the needing to be killed thing. All these bizarre activation conditions people have thought up are just desperate desperate desperation for Lelouch to be alive. Doesn't matter what happened, doesn't matter what the creators say, he
has to be alive. Why? I dunno. I guess it's very important to them? :/