I would say simply change how you're holding it. If you're playing single strap, try 2 strap. If you're playing 2 strap, try sitting down, single strap. The right elbow really shouldn't be under much stress when you're playing; your left, yes, but if you're sitting, playing with a single long strap, then it should be no effort at all on that part of the body. Also experiment with thumb position, you can place it behind the fingerboard, on the edge of the fingerboard, or if you're feeling really flash, keep it free and use it as an extra finger. Whichever way, it shouldn't press on the melodeon treble with any force at all... and if it does you should alter your posture and strap length until it doesn't. Finally, do you push/pull with your right hand as well as your left? If so, try to avoid that, and let your left hand do the work. Some players do (deliberately) play moving the bellows with their right hand, but it's probably not going to work for you.