Re: 'The (almost) everything was not right bike'
Dragray is right. If the chain alignment is right, and the rear sprocket is 'centered' on the rear hub you can get away with a loose or tight chain. If either is installed incorrectly all the chain tensioners, JB-Weld or any other bandaid isn't going to allow you to run without problems.
Center the rear sprocket on the hub and make sure the chain is running in a straight line between the engine drive sprocket and the rear driven sprocket. That will eliminate most, if not all of your problems.
As has been said here many times before; roller chain is not designed to run at angles. It can't 'bend around corners'. It must be straight and tensioned correctly. That includes the tensioner wheel (pully). If the tensioner isn't properly aligned with the chain path it will derail the chain because it will pull it to one side or the other and not guide the chain onto the rear sprocket as it was designed to do.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"