my input:
1. What should I set the gain on the V3 for?
That gonna have to be figured out that day. Consider changing the internal jumpers for a line level signal, or just keep the gain a bit on the lower side.
2. Am I an idiot for using the V3 merely as an AD?
No not really, unless you have an AD1K, then you would be an idiot. It's an extra gain stage but the V3 is not very colored so it should be a big deal.
3. What's the pre on the mixpad sound like? I've heard reports it's a bit noisy...
With no personal experience with that piece, I can just say it's pretty low end and in this world you get what you pay for. I personally wouldn't use one but thats because I have other options. It will "do the job" for you... You want a good signal, too low=noise, too high=drive, find a nice medium in there.
4. Should I just bag this and run a seperate SBD and seperate Mic recording
Only if you can record to the same digital word clock, or feel like spending a shitload of time aligning the tracks in post. I recomend mixing it to tape.
more info for you:
You'll need the mics to be fairly close to the stage mics. If your 50' away there will be a timeing issue that will cause an echo or delay. It occurs less than 50' too, technically it occurs within the stage mics themselves. My point is to get a close to the stage as possable, onstage prefereably. Mixing live to tape in head phone during the show can be hard. Even with nice headphones your gonna be getting bleed from the room. Somethings to think about.