Cassettes would have drain pump, the gradient is to prevent water from flowing back into the drain pan when the pump is off (unit is off).

He will be fine. With the small amount of fresh air, capacity drop max 10%. If he did consider this factor, he would have upsized his calculation by 10%. As for the RH, who cares, this is not precision cooling.

What im interested in is the air duct to another room. Advisable to do that? Meaning, you are sharing air flow with 2 room?

Also, do you have exhaust in the same room you have your fresh air coming in?