bolt definately made out of cheesecake

[Volvi]

mounting rear brake caliper with 2 bolts. Tightening one of them melted in my hand, cap with 1 cm screw in my hand remainder of bolt in housing. Frat. I noticed when removing them they were both narrower in girth (thickness) closer to the top. Dunno how the hell bolts lose weigh on the waist but thats where bolt gave away like soft cheesecake. Dissapointing cant one expect bolts to be tough steel.

Anyway back to prob in hand. What advice can you give in removing bolt from within housing. Yea I realise one way drilling it out etc. tho am afraid of fragging housing thread and dont wanna replace caliper just for that.

So please advise any safe method how to remedy this prob. Cant remove caliper off bike.

Thx in advance.