It's always interesting to read about flutter conversations. There's a lot of misinformation about what flutter is, even in this thread. I'm definitely not an expert, but I did loads/flutter/structures testing for over a decade with the air force and as a contractor. I was also a weight and balance guy for a few years on a modern fighter platform.
Just because a control surface vibrates without any control input does not mean you have flutter. True flutter only occurs in a negatively damped system. You can get oscillations on underdamped and neutral damped systems which don't result in flutter. The F-16 has a known vibration mode on the wings:
Research Paper:
https://arc.aiaa.org/doi/abs/10.2514/2.2696?journalCode=ja
Video:
https://www.youtube.com/watch?v=1DK-zGLK6GQ
While this isn't true flutter, it should scare the **** out of anyone in a GA aircraft, especially an experimental with no published guidance on under-damped or known LCO modes. When you move beyond LCO and into a negatively damped regime, the oscillations increase uncontrolled, and almost always end with pieces of the aircraft coming apart. There's fractions of a second to slow down/pull-g in order to catch a true flutter event before things break. When things go negatively damped, every oscillation adds energy to the system, and it takes as few as 3 oscillations before the magnitude has increased enough to exceed the load limits and break things. All you can really hope for in a true flutter event, is that the aircraft breaks in a way that's still flyable.
As far as changing out the beacon to the ADS-B unit, if vans said it wouldn't need a rebalance, I don't see a reason to argue with them. Just like the entire airplane has a weight and balance envelope, each control surface has a weight and balance envelope. It's not an exact number that can't be deviated from. You can have a theoretical "perfect" surface, or a heavy/light surface, or a forward/aft surface. As long as you're in the envelope the surface won't impact flutter below Vne in any significant way. I've seen control surfaces that were out of limits be approved for full envelope flight after a few more hours of "sharpening the pencil" to determine flutter impacts. If Vans looked at the weight difference and placement of the ADSB beacon and said no rebalance was required, its likely because the change in W&B was so small that 99% of the surfaces out there will still be inside the W&B envelope after the swap. If you've got 9 different coats of paint on there, or for some reason have added a whole bunch of doublers and inspection plates and have a crazy heavy control surface, and then did the swap, maybe its worth reweighing and rebalancing because you might have a fringe case control surface. Chances are though, you have a control surface that was built within spec, and falls somewhere in the middle of its weight and balance range, and adding 0.1 lbs to the aft end of it won't pull it out of limits.