Redbullet

Thanks very much for the explanations, I really understood all and help a lot.
I decided not to replace the fork. Not only that it is expensive, but I think the old one is not yet critically affected. Besides, I would have further issues (and costs) with properly painting a new fork: no too much expertise on carbon bikes in my country. I'll do some preservation works with epoxy on the steerer at the contact with the stem, then I shall add a thin layer of carbon fiber at the contact with the collar, just to restore the 28.6mm diameter (I assume that now, with 0.2-0.3mm decrease in steerer diameter, the collar can not be really tightened on it). And finally, I'll move from 28.6mm stem to 31.8mm and shim adapter. That should prevent further digging in the steerer. I hope the above will solve the issues...

All the best.