For question one, that isn't really true, it just depends on how many are loaded. So if I load five individual M1A1s the "Armor" unit that appears when the tanks are unloaded will have five Abrams in it. It depends on the cargo capacity.
For example, if you want to land a platoon of tanks, you will need to add 4 of those tanks via the edit cargo thing when you load your landing craft or whatever. Then the unit that appears will have four tanks.
As for whether your example is the best way or not*, it depends on what you want to do with the unit. If you are going to land your units and have them fight in one location, it is. If you are going to land your units and then have half of them go one way, and half of them go another way it is not. It would be better to use the teleportation event.
For question 2, I don't think you can but I haven't tried it. The answer is probably no.
For question three, yes, but that is only if you need to have small units move in different places (like you want to have a tank company but not in a single unit). You technically can unload a tank company using the cargo function. But because there are so many tanks or infantry sections in a single unit (Armor #4057 or something is what it will be called) it would only take a single cluster bomb to destroy all of them.
In my opinion, with the way it is right now, the cargo function is really good for helicopters and smaller ships when deploying small teams of infantry, but not so good for ships that can carry hundreds of people and to lesser extent, cargo planes carrying paratroopers.
*This assumes your example was someone adding 100x Inf Section (7.62mm MG/Generic AT) to a ship or something, and then unloaded it and it became a single unit called Inf #6058 or something like that.
Formerly known as Project2035, TyeeBanzai, and FlyForLenin