Okay I've just done a flight this evening after fixing arm #5 and putting the thing back together. The first bit of good news is that the thing flies. Second, and I'm completely THRILLED, is that my vibrations are far lower than they have ever been. MAX vibe on a couple of spikes tonight was .5 and there was a enough wind to keep a large flag on a nearby flagpole horizontal. The video test I did reflects the lack of vibes and is 99.9% flawless. So happy.
I think perhaps arm5 was causing part of the vibration issue and also perhaps using 3M double sided tape for the FC mount and FC, instead of what was provided by XAircraft, made a difference.
Thrilled. I'm making a test video now and I'll post it tomorrow.