The 14bit part is not the problem on 68000, but rather what you want to play. 
Uncompressed audio is easy, and even some compressed formats. But mixing several channels in software might require too much cpu-power.
Yeah, I think people were overlooking that taking paula's 4 x 8 bit channels and turning them into 2 x 14 bit channels would be a bit of a problem for a tracker.
I'm pretty sure none of the existing 8 channel mixing routines were fast enough for the 68000, but that doesn't mean it's not possible. You'll either clip or lose accuracy anyway with that many channels. 4 channels is probably doable and doesn't lose much.
The locking of channels together isn't really a problem, if they are played at the same period then they should be fetched on the same line as the dma slots are tied to the video horizontal refresh. I'm not sure if the fetching at different times is a problem as the way the audio output works is more complex than just outputting to a dac and it doesn't appear to be synced to loading the register.
