RE: Head Washer missing(?) and one torn
Yep. That would do it. A great example of why we should tear our helis apart and reassemble them from a pile of parts before ever spinning the motor one revolution. What? Of course I don't do it. But it is a very good practice, and I've just been lucky in this respect.
I do, however, go over all the screws and make sure they're tight and loctited, and I yank and pull on the blade grips to make sure they're tight and free, and rock them back and forth to check the damping.
If you have no access to another O-ring/damper, see if you can get some windshield washer hosing of the right diameter and cut it to length. I've never tried this, but a buddy on anotheRR forum does, and says it works better than stock dampers.