Walt Wells and the level of OLine development is ultimately going to determine who is the starting QB. We might not know before the start of the GT game, but I bet we know before the end of it.
I think I understand your point, but I think other things will determine who our qb is.....who has a better grasp of the offense, the protections, where to go with the ball/decision making, who the team responds better to, who's more accurate throwing the ball, who can get them out of bad plays, etc.
My guess is that Ainge is right, that no starter will be named, that both QBs will play in the GT game, perhaps a game or 2 even beyond that, until one of them truly separates himself, which is something not likely to happen until both have gotten meaningful snaps in games that matter.