yep which also shows that segwit solves nothing. it cannot force pools to only accept segwit keys meaning native keys can still mess with the base block in MANY attack vectors that affect the network
Yes, they can fill a block if they outbid everyone on tx fees. They can do that right now too. They can do that with any blocksize. Segwit users can do that too. Nobody quadratically spam.
Miners can also outbid by giving up the tx fees they earn in a block and mine an empty or full block if they want too. They can always do that