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.
I've tabletop tested it. They would just need to make enemies using 5e stats and use XP split so 6 party members wouldn't be level 5 by the end of the surface portion. 6 level 4 characters using proper stats would balance combat out a lot more. Some enemies like the Gith Patrol actually wouldn't need any rework. They're already tough enough.
As for the beginning, they have made enemies too weak for multiplayer. 4 players + Shadowheart against 3 proper intellect devourers would be a solid challenging fight. 1 player + Shadowheart against 3 is insane, so they nerfed them. This is true for imps as well. So, I suggest they again stabilize it by making players start with 4 custom characters and/or origin. So your base party size is 5 or 6. The customs are already defaulted so if you don't want to tweak them just alter the first, accept the rest, and start playing. Don't like the customs? First chance you get, dump them at camp. They can simply be your tutorial characters.
This would make it so multiplayer and single player are pretty much the same and monsters are balanced for both,