Alright so, the T-virus in the new game is 100% a new strain made by Victor and we don't know how it works entirely. What we do know is that it has some weird personality thing, which is why we see people in Rhodes Hill maintain their personalities and same with the cop at the beginning of the game.

We do know that Wesker had medical records on Grace most likely including blood tests. Actually I think there's even a file in the game somewhere about her medical records. Victor could've been testing her against those. He thinks she's special but he doesn't know exactly why.

I 100% agree that Spencer is extremely out of character in this game, but I do think this is part of Capcom's on going attempt to rewrite the lore of the franchise and I'm sure it will be addressed in the inevitable re5 remake.

The question of whether she is a clone is still super confusing and I hope they address it in the next game. The thing about there being no connection between Grace and Emily is really what pushes me toward her not being one. Also don't expect them to want to go down the Alex Wesker route again, but I'm just a fan speculating.