So the issue, if I understand it correctly is just that you lost functionality on the iPad? the stagescape M20d continued to function as did the speakers? You just lost control from the app?
If so that's no reason to lose positivity of Line 6. I would have thought a sound man's first priority would be SOUND, not whether or not he can faithfully control the stagescape from his iPad.
What is he doing anyway that he has to continually fiddle with the controls all night. I will admit that if you are maintaining wifi but losing functionality, it's an issue that needs to be addressed and Line 6 needs to know about it so that they can work to fix it. But I don't know that many of us would run into that problem or notice it if we did. Personally, I may use the ipad to go out front and do a virtual sound check. And throughout the night, I may make an adjustment or two here and there, if things change as the room fills up with people. But I'm on stage at that point and I just go straight to the M20d and do a quick tweak between songs. I sure as heck wouldn't do a restart of the M20d during a set just so I can play with my iPad. That would wait for a break. If the sound needs to be adjusted that badly in the middle of a set, I would suggest the sound check never got completed, which of course can happen. Sometimes, you don't even get a soundcheck.
Of course, I'm just throwing all that out there in good fun and out of curiousity. Getting back to the real issue, is what may have caused it. As I said anytime I mess with the ipad app, it's never for extended periods of time like that. IF there is an issue it's in getting the two to connect in the first place, but once they're locked they are locked. I had heard that an issue that can prevent the app from connecting to the M20d in the first place is if there is another wifi network going on in the background. I wonder if it's possible that another wifi network could cause problems even after your made a connection with the network you are using to link the ipad to the M20d. Although if it took a restart of the M20d to fix I don't know that this would make sense. Unless of course you didn't try just turning the wifi on the ipad off for a bit and then turning it back on. I don't mean just closing the app and re-opening, I mean going into settings and turing wifi off then turning it back on and choosing your network again. Turning the M20d off and on would essentially do a similar thing as it would remove the network for the ipad and then allow it access again. Not identical, but close enough that it could resolve the same issue.
If it is something like this, I don't think that there will be a fix via firmware or app updates. It's just a fact of life. You may have to figure out a way to make a stronger wifi network to make the connection more stable. It could be totally something else, I'm just thinking out loud here.