Any mechanic that cannot break free seized rings/bolts isn't worth going to. I'd try somewhere else. If I were closer I'd come by and help you for free.
Absolutely agree with the above!
Yes, they're probably partially seized through corrosion, but a good soak with Duck Oil, Plusgas or even good old Diesel fuel is likely to penetrate sufficiently. The rings on adjustable coilover units are usually shallow and of large diameter compared to the depth of their threads, so it might be that even removing the leg and holding the ring (gently does it..) in a decent bench vice with cork or aluminium jaw pads would get the rings free.
Worst case scenario, the rings could be cut with careful use of a hacksaw then prised off, new rings are available for all of the good brands of coilover. I use AVOs on my track cars, but have fitted Maxspeedingrods, Koni, Spax and BC units with success and have always managed to get hold of parts for these things.
I also wonder whether the mechanic you asked may simply not have wanted to take the job, in which case some folk will feed the customer any old BS rather than just admit that they'd sooner spend the day doing routine servicing work!