My heart bleeds for you Gman. I could have so easily have been in your position. I had problems with various things that needed "fettling" after we bought our Becky - 2010 Panda Dynamic Eco. Sorting out the very noisy front brakes was one. It might amuse you to read what I posted on the forum at the time:
https://www.fiatforum.com/panda/459983-beckys-brakes-few-surprises.html?459983=#post4339097
Apart from the problems I had getting the bottom pins out I also found the top pins would not unscrew. I must have been so near to snapping one off, just like you, I prefer not to think about it. In the end I decided to leave them alone when I found that I could gently slip the end of a small flat bladed screwdriver under the pin's rubber boot and work it over the end of the pin. Then you can simply slide the caliper off the pin, leaving the pin in place. I did this again last year at service time just to clean up and lube the pin.
It frustrates me that I can't remove these pins but I'm getting on fine doing it this way so I'll probably just leave them alone unless/until the pins corrode and need to be renewed. When/if that time comes I'll be applying plenty of heat and shocking them with my special Vibro Tool. At least the carrier can be removed and worked on in the vice. The pins on my Ibiza screw straight into the one piece casting of the hub itself so repair is a considerably more difficult operation - and much more expensive if you need to buy a new hub! - I have noticed on mine that the drilling for the hole the pin screws into has been drilled so deeply into the carrier that it's just broken out at the other end which leaves me a wee hole which I am squirting Plus Gas into from time to time in the hope that it may help if I ever need to try to undo it.
I'm sure the pin will be "hard metal" high tensile steel probably, so not easy to drill. I'll be very surprised if the garage can't sort it for you and would make a guess for you at a cost around the £40 to £80 depending on what their labour rate is. Of course I'm probably wildly out with that guess so please do let us know how it goes and, if you don't mind, what it cost? I've got my fingers crossed for you that all goes well and doesn't cost too much.
regards
Jock