Englishal,
It seems to me that your examples cite aircraft, which I would agree, sound to not be "airworthy" (conforming to their type design, and in safe condition). My argument does not extend to unairworthy aircraft.
When I rebilt a Thorpe T-18 years ago, the fine fit of the stabiliator pivot was of critical importance. I used an adjustable reamer to assure a bushing fit of better than 0.0005" tolerance, so as to prevent a loose fit there being a source of flutter.
Yes, if a flight control is out of balance, or a bushing is loose, flutter can happen very quickly, and with horrendous results. For anyone who has not seen it, allow me to link the following....
As for airspeeds, I cannot speak to limitations for Vans aircraft, other than to say that I am not aware of Van's aircraft having demonstrated compliance for certification - but I could be wrong.
For certified "slow" GA aircraft, operating at their normal altitudes, the following Wikipedia passage would describe why Vne in IAS is Vne. A pilot does not have to worry about exceeding safe maximum speeds, while still flying at an IAS of less than Vne.
"The IAS is an important value for the pilot because it directly indicates
stall speed and various airframe structurally limited speeds, regardless of
density altitude."