Hi James,
Mavlink just has a fair amount of overhead attached to it. For direct control I'd think a simpler, more direct protocol would be more appropriate. But that's just IMHO. It shouldn't be hard to try it out, like you say. I'd at least disable most of the other Mavlink telemetry messages so they're not clogging the airwaves (except heartbeat of course, maybe STATUS).
I've definitely seen lots of data errors and re-transmission attempts when using BT to, for example, refresh all the parameters. Reducing the baud rate between AQ and the air radio should help with errors, but of course slows down as well.
If you don't have an actual radio configured in AQ, you could just stuff the radio.channels[] array with values from Mavlink messages. It'd be a quick & dirty way to try it out, at least. With um, lots of bench testing before any motors get spun up
There's some discussion about USB solution in the M4 forum... there's not much out there though, except one $60(?) device for USB to PPM, which you'd then need to plug into a Tx module. Or maybe it already does DSMx... I forget
Yea, we need help that's fer sure.
Feel free to jump in anytime once you get oriented. With QGC especially, lots of room for improvement!
Cheers,
-Max