A trick you can do to keep the idler from twisting is drill a hole in the mount for a set screw, or drill into one side of your chainstay tubing and make it a lock screw. 1/8" hole is plenty and any screw jammed in will keep it from twisting into the spokes.
If you know someone with a tap&die set, can tap a bigger hole in the mount and thread in a set screw. But a little hole is better than no hole for a set screw.
Most engines will vibrate alot at a certain rpm. My winter bike vibe's like crazy at around 3500-4000, but smooths out after around 4500, opposite of yours I guess
nice build, and gl.