Question:

The rules are fuzzy on when general supply is traced. I am assuming this is done after reinforcements are placed, and only at this time. An airdropped German supply depot would not provide General supply until the beginning of the next Norwegian player-turn. Correct? Also assuming all units in staging boxes/at sea are in general supply. Correct?

Answer:

I see no rule that requires general supply to be traced at one time and only one time per player turn. Instead, unless I am missing something and am not recalling my playings of the game correctly (both are possible), general supply is traced as needed. If you are about to do something with a unit, you check its general supply status. For example if you are about to move a unit in the movement or exploitation phase, you check its general supply
status at that time. If you are about to attack an enemy unit, you check its general supply status at that time.

This situation is similar to attack supply. Note that a German depot airdropped in a turn can be used for attack supply. It is also available for general supply. Note that since the air phase comes after the movement phase, air dropping a depot won’t help put German units into general supply during the movement phase. However, if the depot is still present during the exploitation phase, then German combat/motorized units could use it for
general supply.

Also note that Rule 11 specifically gives an example of a British unit going out of general supply DURING a German player turn (presumably as a result of German actions in the turn).

Finally, note that going out of general supply has no effect on a unit until its second [game] turn of being out of general supply. Thus, the only practical consideration here is if a unit that is already out of general supply gets back into general supply in a turn. Example: A German c/m is in its second game turn of being out of general supply. During its movement
phase, it has its movement rating halved per Rule 11. However, it moves so that it ends its movement phase in general supply. During the exploitation phase, if this is still the case, the unit is now in general supply and has its full movement rating for exploitation movement. (There is a gray area here: When the c/m moved back into a region where it could trace general supply during its movement phase, did it regain its full movement rating? Answer: No.)