Oh dear. There's no easy solution for these unfortunately. Usually the link pipe unions are seized as well. I send them to
BCS to get the screws and unions drilled out on a CNC. BCS can't currently make the replacement link pipes, but they are easy enough to diy if you have a flare tool. I cut the dead link pipes off before sending them away to retain the rubber sleeve on them which I refit to the new pipe before flaring.
Their workshop is shut down at the moment, but they are offering
20% discount to anyone willing to pay in advance for work when we're allowed out again.
I would suggest that you leave that area well alone for now until you know for sure when you can get the calipers back, otherwise the car might be down for many weeks.
Side note: I have read of a technique using ammonium aluminium sulphate to dissolve steel from aluminium but I haven't tried it yet. I will be attacking some pinch bolts with it as soon as I can get hold of some and will report back on what it does. If it works I'll try it on a scrap caliper too.