i havent really heard about too much of either of these things, other than they both just sound like fan theories to me. i think i saw a single post a few days ago about pearl leaving and getting replaced with someone named ‘topaz’, but the post i saw was quite obviously a prank (the ‘source’ link lead to a gif of amethyst-as-pearl making fart noises or something, so it clearly was meant to be a joke),
sometimes fans are right about their theories, but i find it a lot less stressful to just accept fan theories (especially the angsty ones) as just that, enjoy them for what they are and appreciate OP’s clever thought processes, and just wait and see what happens in the actual show! you can enjoy both headcanons/theories and the show canon separately and simultaneously! that’s the beauty of fandom! not just SU but any fandom. especially gravity falls fandom. jesus guys i think you need to give your conspiracy-theorizing brains a breather or something. i am worried for your health.
and, worst case scenario? even if some of that stuff DOES end up happening, i think the writers of this show have proven themselves to know how to balance out the feelings in an episode, so you’re never just ANGSTBOMBED and left feeling at a loss. audiences need breathers, too, and these writers know that! whatever happens, i highly doubt it’ll be like tumblr, where fan speculation can sometimes feel like all angst, all the time. doubly so when the show is somewhat cutesy, lighthearted, or aimed at kids (steven universe is definitely always the latter, but often times all three at once) because fans tend to like to fill in the gaps in source material with their fan content.
tl;dr: dont worry friend! it aint over till the last episode airs! (even if pearl DOES leave she might come back! even if greg DOES die trying to save steven from something i doubt it will feel like a total loss! it will all be ok)