GWX Dev Team
Here's how I'd answer your questions based on the 2nd Edition rules:
1. When the Allies perform a paradrop on the first turn, they may unknowingly drop on hidden units. If the Allies are willing to anticipate the location of a hidden unit that is adjacent to or directly on its drop hex(es), can the Allies assigned a TAC to ground support for that anticipated combat?
1. The Allies always allocate TAC counters to the "Ground Support" box on the Aircraft Mission Chart (AMC) before invading (see §3.5). If you anticipate a hidden unit under your planned paradrop sites, this is when you make TAC available to support them, although the TAC will be available for ground support of any attack within TAC range, not just for the paratroopers.
2. The Allies place their invading units, including paratroopers (see §3.6).
3. The Germans reveal their hidden units and then the first combat phase begins (see §3.7).
4. In the combat phase, the Allies allocate the TAC units in the Ground Support box to any combat within TAC range, including allocation to help any paratroopers that survived their drop onto an enemy unit or into an enemy ZOC.
2. The rules don’t seem to indicate a sequence for Naval Units’ gunfire. Can the Allies withhold assigning naval gunfire to combats until after the Germans have revealed their hidden units and the locations of all hidden units are certain?
I think you are correct that the rules are ambiguous on this point. I usually place the naval units after the Germans reveal their hidden units on the grounds that warships are highly mobile and could have redeployed at will during a week-long turn to wherever they were needed within an invasion zone.
Naval units are allocated to specific combats during the combat phase (see §19.4), which occurs after hidden units are revealed (see §3.7), so if you play that the Allied player must place naval units on the map prior to the revelation of hidden units then the hidden units will have already been revealed by the time combat starts. You can then add the naval bombardment factors to any attack against (formerly) hidden units within range of the naval unit(s).
I hope this helps!