I noticed there are 2 different scenarios when attacking units carried in air transport (on land and not in cities):
1) I kill the air transport and everything inside die!
2) The units inside can actually fight and the air transport gets killed last!
what is the mechanism behind this?
As far as I am concern, if you move your units to AT and meanwhile, your AT is getting tbed, all your units will act like the part of AT and killing only AT will be enough to kill all units. But if you just move units to AT and somewhat don't move it, all units will act like land units and their defence will add up.
Here is the small video showing this. Enough movements have been done to make it clear AT will be tbed.
You cannot do anything against this kind of backstabbers, best solution is, as u mentioned, leaving 1 unit in each city. Allycapping is just meant to let you give some of ur lands to ur ally without having war and is useful in team games. Next time make sure to either find genuine ally or leave units in each city
It is actually possible. If you send unit and hold it on the stack that u wanna attack, you will see its stat. Similarly if u wanna see your stat when u attack somewhere just use same tactic after attacking