My answers in bold beneath each question.
I just have been getting into the game, and have to admit it is a bit overwhelming. But still fun!
There are a few questions I have that I was having trouble finding the answers to by searching the forum.
1. Support units (specifically naval and aviation squads): How do I create these and move them around? It seems I have a shortage at my bases, and since I'd like to move my aircraft around to new bases, that shortage seems likely to multiply if I don't figure out how to move them!
You do not create any units at all. As far as naval and aviation squads go, you just have to look to see what units have them. You will be looking at the various kinds of base forces for aviation support, and some have more than others. Naval support is present primarily in naval base forces. there are also 6 small units with 'detachment' in their names (which start with A, B, C, D, E, F) that the Allied player gets which have a little air and a little naval support in them. They are very useful for helping to get island bases started because they will help you to unload very quickly. Don't leave them anywhere permanently, move them around and use them to help you get unloaded at new places.
2. Level bombers early in the game for the allies: are these good for anything? I'm especially thinking about the Philippines. I don't want to waste my fighters escorting the bombing land bases--if they are escorting bombers I want them going against naval assets. I assume level bombers are next to useless for that.
It depends on the bomber. 4EB are very little use against naval targets, but they can him some (as one very surprised IJN destroyer captain found out IRL in the Solomons when he elected not to take evasive maneuvers when a high-level B-17 raid attacked as he was along side a damaged vessel). Train your pilots and they will be much, much better at whatever they are trained for. See various threads on the forum.
3. Replacements (pilots and airframes): it seems from reading the forums a lot of people want to turn these off. I can't figure it out. Sure the replacements may be green, but isn't that better than being shorthanded?
Replacements are very precious at first. Turning them off by default is very helpful because then you won't have a zillion units all drawing replacements and emptying your replacements pools. You then go around and turn on replacements in just the units you want (air and land). That way, those units that are in critical places will be more combat ready. If your replacement pools get used up by all the units in the rear area you will have problems! Of course, over time replacements will build up and you (likely) will not have that problem, except perhaps with some airplane types. But that will take a while.
4. Convoys: My strategy is to set up a series of major collection hubs throughout the world (eg. San Fransisco - Hawaii - Australia - India). Major convoys would run between these bases to keep their supply very high. Smaller convoys would then run regionally to keep the bases supplied from these hubs. Is this how experience players organize their convoys? Also, I've figured out how to set up computer controlled convoys that go one way (eg. to Hawaii from San Francisco with fuel, then back empty). But is there a way to do this two way?
Pretty much. As time goes on you will refine/change exactly how you do it, but that is the basic way. Be wary of having big convoys getting too close to enemy territory!
5. How do you know which bases have a shipyard?
Look at the base by opening the base display, or do a mouse-over of the base (much faster). There might be a way to list them all, but I don't recall.
6. Is there a way to turn China over to the AI? It seems cheating to be able to coordinate my efforts with China when certainly that wasn't the case historically.
There is now way to turn any areas over to the AI.
Hope this is helpful.