Support & Motorized Support provide the same function, basically to 'fill out' the support needs for other LCUs in the same hex, allowing them to fight, recover & replace more efficiently. the real difference is that MotSupp moves (and un/loads to a TF) as a vehicle, not a squad. S & MS don't help movement of supply points, there's no point to stringing them out along a supply-path. keep them stacked w/ LCUs, distribute them as needed to keep LCU support #s 'in the green' when in combat or when resting/training/rebuilding.
Naval Support is a different animal. it's only of use in a Port hex (or in a hex you are amph-assaulting. It aids primarily in ship un/loading, also i believe they'll help ship repair and ammo reloading by adding capacity to the port. not entirely sure, but 2nd Engr Amph Bde sounds like the kind of unit that's a big help to an amph assault, esp. against an atoll. (atolls generally have limited stacking capacity, & an invading force will automatically attempt a shock attack. so the Amph Bde helps by increasing unload speed of other units (once it gets ashore), almost as important is its help in reloading the invaders back into their ships so the LCUs don't eat overstack penalties. note also that for quick-un/load amph assaults, you need to use APA/AKA/LST types, w/ a Force HQ embarked on an AGC in the Amph TF.
Air Support is also a different animal, it's needed at land airbase hexes, to keep the planes repaired.
& welcome to the forum!