I have also owned a .177 '100 for a year or so. I must admit that your horror stories have certainly conflicted with my own experience of my ex gun. I free floated the barrel and also fitted the A.T modification kit. As another poster has mentioned, I always found the gun to be excellent with my own best grouping of 8mm at 40 yards indoor. In fact, I probably recorded higher average HFT scores using the HW rather than any of my Steyrs.
It looks to me like you have been unfortunate in buying a rogue gun (well, rogue barrel.) There is nothing more frustrating than discovering you have bought a dud, and it is clear that you have tried everything to improve the guns performance. Simon has provided us with an example image of what a "good" finish to the barrel should like. The image of yours clearly demonstrates that whoever was responsible for making yours was having a bad day.
I might be inclined to part ex the gun, although the fact that you clearly have a conscience will make that difficult!
Anyway, good luck.