I’ll elaborate a little further…
Is it possible to load two units, move two zones into an enemy coast zone and start an amphibious assault in one turn? I’d say yes, but i am not sure about it.
Yes. Since an attack is involved, this would be done in the combat movement phase.
And would it be possible to load move two seazones and unload into an friedly landzone in one turn? or do you have to wait one round because the transported units count as moved?
Yes. Since no attack is involved, this would be done in the noncombat movement phase.
It’s important to differentiate between the movement of the transport and the movement of the land units. The land units’ entire move is used by loading onto and/or unloading from a transport, and they may do one or both in a turn. However, if they do either or both, they may not also move in their normal way, either before or after being transported. The transport’s movement is affected by the land units only insofar as its movement ends (including loading or offloading other units) when any land unit offloads from it.
Of course, there’s more to transport and land unit interaction than this, but this covers the very basics. External factors, such as the presence of enemy ships and engaging in combat, also affect how transports function.
Our second questions is, wether an Anti Air Gun can shoot only once at attacking air units or, unless the attacker does not retreat, once for each combat circle? As far as I got it, it can only shoot once in the opening fire phase and then be replaced on the normal gameboard.
As A44bigdog pointed out, you’re correct.