I don’t know of a better place, so I think you’ve come to the right spot. (There’s a lot of discussion going on on Discord these days, but it’s a much less easy way to get such discussions going.)
The Keys definitely can/do apply to other PCs. Why not? That’s great material! The Keys are basically the players’ way to tell you, the GM, what they’re interested in. So if they want a major part of the game to focus on how PC A is in love with PC B, that’s exactly what it’s for!
As for buying a Key you’ve bought off again, I think it’s a bit of a simplification. We want the buyoff to be significant, and to present a permanent change to the characters. A major shift, dramatically, and for it to have weight, it should have major and/or permanent consequences.
As long as that principle is intact, I can see bending this rule to reclaim a Key once more. There will be some stories and some characters where returning to a bought off Key might feel totally right (especially if lots of time has passed, and there have been major consequences), but generally speaking it’s not of interest, so it’s a good base rule.
I would have ZERO problem with taking a Key like the Key of Love or Key of the Mission or Fraternity or Faith (any key which has a specific subject) multiple times, with different subjects. That sounds great, too!
Ultimately, it’s a question of what you consider interesting as a group. If the players treat it like a boardgame, where they are trying to score the most points, the system might still work, but you won’t be getting the most out of it.
The players should think of the Keys as a way to signal what interests them most about the story and their characters, and the GM/Story Guide should be using the Keys to prep their material. Use each Key to offer an interesting choice to one or more characters; it’s a really efficient and powerful prep tool!
If both sides are doing that, your game will rock pretty consistently.