Luffy fought BMs no.1 previous arc, there is no point repeating that plotline again, when the antagonist and the man they planned to take down since entering the NW, is quite obviously Kaido (who is now allied to BM, so both can be taken care of). I honestly dont think there even is an "Ace", we got to see Kaidos top 3 already. Another fighter above King would make them too powerful compared to the other yonko crew we saw previous arc, so the current stalemate would make no sense.
It will take an extreme battle for the tag team (whoever is part of it) to win even at full power, but you want to put them in difficult battles before the main ones? No, Luffy and Kidd are going through fodder straight for Kaido, and BM is going straight for Luffy, he wont have time fighting commanders.
Regarding the tobbi roppo, i still dont see how the weaker Strawhats are gonna beat them. Some of them could, but it doesnt mean they will, there are tons of other matchups Oda can put them in. This is literally why there is an alliance full of strong people here with them. Because it will take all that to take out the top fighters of BM and Kaido.
And i still dont think we will get all these 1on1s you guys predict. Nami for example has a power suited for staying in the background and creating powerful attacks that can take out many enemies. Pinning her on one enemy the entire time is a waste of her potential. Same for Usopp. This is gonna be a full out war, so one person can fight more than one enemy at different points of the war, plus we are probably getting another force led by Neko joining in the middle of it, Kaidos supernovas turning on him etc. There is tons of ways to write this, them entering and then everyone getting a 1on1 is too boring for a war of this scale