Wouldn't worry about that at all. File in your "least of my worries" folder.

I would only use ny-lock nuts IF AND ONLY IF I had easy access to the bottom of the engine mount rails. Typical trainer uses what I call "cowl cheeks" which gives you great access to the bottom of the mount rails. Some mounts do not have flat bottoms so nuts will not rest flush to the mount material. If you don't have access to the bottom (cowl) or the bottom is not flat then get a tap for the bolts. There's a nice kit that has 2-56, 4-40, 6-32, 8-32 and 10-32. Of course, 1/4x20 is NOT included. The kit includes the undersized drill bit and the tap. Get a pin drive or T-drive that will accept these "bit" sizes.