Well, the 2D sounds will most likely not be used as often as the 3D sounds, so why not taking those for the ambient sounds. Calculating balance and volume should be pretty easy. Only the doppler effect would be missing.

I'd also ask JCL about that sudden falloff you were talking about. I never noticed that, but it also has been ages since I placed sounds in WED.

Last but not least I'd just try it. 16 players are much, but who knows if the omitting a sound is even noticeable. Even if so you do other tricks like cutting silence from the end of the sample to make it shorter, mix sounds together, e.g. three shots in one sampe etc.