1) Fact.
2) While it is possible to visually identify spring damage, I find it hard to accept that the damage is characteristic of one cause only. Also note that dieseling can be caused with light pellets, so how can you tell?
3) Fact from personal experience. In my youth. cautiously experimenting with deliberate dieseling using a Webly pistol. I found that, while it clearly increases MV, it greatly reduces accuracy. I stopped my experiments when the gun re-cocked. OK it does not take much to re-cock an old wobbly Webly pistol spring, but it surely did happen.
To the posters who reason that a light pellet will give way before a heavy piston, note that compression tube pressures act equally in all directions, and that the larger area of the piston will will therefore cause the piston to experience more force than the pellet. Also the pressure in the barrel will in fact be lower since there is restriction in flow at the breech, and also possible sonic shockwaves there too. It is clear that even without dieseling, piston rebound can occur (many of the tuners on here will attest to that) - the rest is a matter of extent.
True freedom includes the freedom to make mistakes or do foolish things and bear the consequences.
TANSTAAFL