2. More characters to control means longer combat rounds. (You might say, but you will kill the enemy quicker. Well obviously there will have to be more enemies or beefier enemies to balance things. Hence longer fights.)
If you have one thing that decreases fight times (killing the enemy quicker) and another thing that increases fight times (stronger enemies), each by unknown amounts, it's impossible to say whether the total time will increase or decrease.
Additionally, assuming you have <6 players, each player will control more characters during combat. So each player will be actually participating in combat for the same (in the case of more enemies) or a higher (in the case of stronger enemies) percentage of time. In the latter case, combat will feel faster as there will be less time spent watching enemies take their turns. This ties into your "benefit #3."
with increased party characters, they may need to rebalance combat too. with more enemies and stats changes. larian formula has always been bullying the players and force them with abusing terrain/elements and barrelmancy. that's their winning formula. also.. the number of encounters are limited.. very limited and no random encounter. so i would say it make sense to really makes each fight memorable.