The thing with Arms is that you're mostly strong on aoe fights with 3+ targets and on Execute phase ofcourse. When the boat phase starts, there will be only 2 bosses on the main platform since the 3rd one goes up. Going on the boat will allow you to use Executes. At least that's what I think it's Sarri's point.
Bingo. As I've so nicely said in the past, Execute is always the answer (unless it's Bladestorm). You'll deal more damage on pure single target over 1:30 if you get to execute than you'll do over the same duration against two targets without Execute, RNG can of course change this in either direction.
On Maidens in todays gear there should be no issues getting to the last phase with only two boats. If you only do two boats you won't have to worry about the CC boat, and you won't need to waste your cds on the boat, meaning you can save all of them for the execute burn.