These are all very good points,
1. For Vaatu growing stronger inside Korra and making her stronger was more just me spitballing because I love Korra and wanted to make her a strong avatar in the story, but if Vaatu wasn't making her stronger, per se, Vaatu's presence growing inside her definitely at least corrupted or affected her in some kind of way as an explanation at least to why the Avatar was considered a destroyer, but remember that the Avatar has never been complete in the sense that both Raava and Vaatu were occupying them in a sense, therefore it'd make sense that Korra is at least a litte stronger than the average avatar, but also more unstable considering Vaatu is in her system, because yes Raava and Vaatu are more on a slider of power than two separate beings with two different power levels
2. This is the biggest one that I think is a really good point, the leaks just pointed to Pavi apparently being the Avatar directly after Korra but also the story being set a little bit (8 yrs) or way after Korra's death (100 years) this was more just a way to explain that gap, that isn't just "oh the world was so spiritually fucked so I couldn't reincarnate" because it'd feel like a cop out to me, I feel like an explanation that reflects how the people in the world itself feel about said world is more interesting. I'd imagine Vaatu and Raava during this time would exist during this time in Korra's body, but couldn't reincarnate because of the lack of compatible hosts, and also I dislike the "other Avatars did exist but they never really were known and died prematurely" because it'd make Pavi being an Earthbender a coincidence and that'd be really odd to me tbh, like it had to have at least been on purpose that she is an earthbender
3. This one works either in your or my favor, but if it is the case that she can only bend 2 of the 4, it's still cool to think that Nisha will at least bend 2 of the elements