That would make Ella purer than the vast majority of the cast, including the Chosen, which goes against her own beliefs and she is the resident expert in compatibility thanks to those eyes. I don't think I'm underestimating her Body compatibility. I think that people haven't internalised this. I'm leaning towards a 70-30 split at best.
1. I'm not saying she actually is 90%, I was just asking what you're basing your estimate off.
2a. Where do we find out the compatibility of the cast and the Chosen?
2b. It might not have been you, but I know someone said Valravn has 100% compatibility. In case you're going off his monster report, I'd just like to point out that the "7(100%)" is under "Origin." It seems more likely this refers to his lineage makeup, not how compatible he is with a given Apostle. So Valravn might have, say, 50% compatibility with Darkness, but his Origin will still say 100% because he's Apostlespawn. So if Ella monsterfied, her monster report would say "3(50%), 4(50%)" I think this point is bolstered by Deryl's monster reports. Deryl Tertius has 1-12 (%???) while Deryl Rebis has 12(91%), 3 (6%), and 4 (3%).
Alternatively, this might actually indicate to compatibility, but only because when you monsterfy, the trait completely takes over and modifies your genetics, altering your compatibility.
3. Even a 20% compatibility gap is significant. If her compatibility with Memory is 30% and MC's is 49% at best, that 19%(?) gap still caused a meaningful difference in their ability to use Memory.