Re: riddle me this...
I will jump in here to answer your two questions.
Chain stretch, probably not at least not after a very short while. There are 2 things that can cause the chain to loosen, engine twisting and engine sliding. If you installed the tie bracket between the rear motor mount and right bearing plate that will eliminate the potential for the twist. So the only other option is it sliding. You say that the clamps are resting on the water bottle studs, so that eliminated the clamp from slipping but that doesn't stop the motor mount plate from slipping slightly on the clamps. It takes very little movement for the chain to go from bass string tight to loose. This is the reason we just came out with the chain adjuster which also helps support the motor so it can't slide any more.
For the chainring bolts. They are M5 1.0 and stock they are 22mm long so you will have to figure out length for your application.