I agree, their manuals aren't geared towards beginner builders but then again, their aircraft arent trainers.

Blue locktite on any metal threading into metal is a pretty common practice.

Given that its an electric plane (hence very little vibration), my experience would lead me to speculate that you either have a bent motor shaft, out of balance prop, or didn't tighten the bolt properly to begin with. Also since experience is something you tend to get right after you need it, I'd also add that I fully inspect any airframe after its maiden flight for loose hardware.

I've had nothing but spectacular customer service and have owned several airframes from Extreme Flight.

I'll also add that if Curtis said he was going to forward your concerns to Chris, that there is no doubt in my mind that he either will or has already.

Edit to add since I didnt read the entire original post at first:
Since this is a pre-owned plane I would also be double sure to carefully and fully inspect any work done by the previous builder.
Personally I'd be more agrivated with the first owner than the manufacturer.