@Driel310 have you worked out the answers , now you have seen the rules ?
Can buy as many Sea Mines as you like , but only one can be placed per nation in each SZ . (Unlike Land Mines , they don’t have a facing , but protect the whole SZ.)
America pays for the two (maximum) FF units from its own “income”.
The advantage being that they can move and attack the same turn, as they are placed in Phase 2 of America’s go, before : Combat Movement.
Britain’s player can, indeed, buy Sea units for its 3 x 12 IPC spends.
With regards to Germany’s choice of targets , It looks like you choose your target after you hit . (Found at the end of the rules , with the unit descriptions). You must remember, however, to roll the AT and Stukas separately.
As different units roll with different numbers , you would not lose out .
I suspect it makes most sense to use AT guns first , as they can only target Tanks .
A Stuka could target Art (if Tanks had been killed ).