That's a great idea. But, I think the backline pets are randomly spawned for each battle, but not 100% sure about that. If so, quitting, loading your important team and restarting could result in worse backline than before.
No, once you are in the scenario the team/s you face will always be the same. Even if that wasn't the case, however, having a 3-member team of redundant pets as a soak team for every battle could not possibly hurt, since the reduction is 10%hp per battle, so you can use the same 3 pets the entire way through the dungeon. That is not the case, though. Once engaged, the pets you face are always the same and my comment stands.
I tried that. The randoms can change so much, though; from critter/undead to mech/mech, and so on, that a soak team is the easiest and best solution. It takes a bit more time, especially for those of us who rely on Rematch, but, it's easily faster overall.
When going in to any of these battles, use a disposable team setup to soak the 10% hp loss of cancelling each battle. That way, you can load your soak team, check what the backline pets are, then cancel and load your main team appropriately.