1. can air unit retreat alone (not all units retreat) in the combat?
No.
2. if all seazone is occpied by emeny around the sub, how can it retreat? can it stay at the original sea zone? in this case, is any diff between attacked by air unit and naval unit?
It can’t retreat, unless you are playing with the optional submersible subs rule, in which case it could submerge.
3. when amphibious assault, if there are enemy naval units in that sea zone, does the attacker transport (/w cargo) invlove this sea battle?
Yes, it could be destroyed (along with the infantry on board). You have to resolve the naval battle first and the transport has to survive to be able to unload the units into the amphibious assault.
4. if there are sea battle before amphibious assault, only the land unit(unload from transport) can involve the land war? or i need to decide the air units (no involve the sea battle before) to attack that island in the combat movement pharse?
That is correct. The land units from the transports can participate if the transport survives the naval battle, and you designate which air units are in the naval battle and which are in the land battle during the combat movement phase.
5. if all chips and units on the board, how i purchase more units(using sth to represent it), or this is the offical limited, to control the number of units?
Just go get some more poker chips! :) Supposedly you can’t. However, we always thought this was a stupid rule that has nothing to do with the game - we have pieces from two games combined into one. :P
6. if i got back the place which original belongs to my ally, then now, the place is belongs to me, or back to my ally?
It belongs to your ally.
7. in amphibious assault, can the attacker retreat? and how to do? back to transports?
Land units can not retreat - they must fight to the death. Air units can retreat if they are all that is remaining.
8. after amphibious assault, is the battleship legal to move after the “one-shot” attack in non-combat movement phrase?
No. The bombardment counts as an attack so it can’t move. This also means that it can’t fight a naval battle and bombard on the same turn, for example during an amphibious assault where the sea space was occupied by the enemy.