Brake boss unscrewed

[Ajax Bay]

I am getting an Islabike back on its wheels and on the road/cul de sac (5 year old rider) after previous owner neglected it.
The bolts holding the V-brakes on are disfigured by rust (along with chain, RD, all 40 spokes - you get the picture).
One bolt came out perfectly normally but unscrewing the other one resulted in the complete boss unscrewing instead.
I need to separate the bolt (27mm long: far longer than necessary imo) from the boss, so I can screw the boss back into the frame.
I could get another boss and then cut off the bolt head and extract but I'd like to avoid if there's an alternative solution.
Have not had this issue before so any suggestions, ideally based on experience, would be appreciated.

 

[Ajax Bay]

 

[Cycleops]

You could try holding the boss in a mole grip or vice if you have one but if it’s well rusted in it might be a losing battle. Buying a new boss and bolt may be the only answer.

 

[FrankCrank]

Applying some heat may help to separate the two. Some sort of nutlock compound might have been used on the screw, and on the boss itself for that matter. The boss part usually has flats on it to aid screwing into the frame? Good luck.

 

[Cycleops]

Good point FC, they always use thread lock on the bolts.

 

[Ajax Bay]

Thanks all. I have tried mole grips but there's only about 2mm to grip on and the boss has no flats, unfortunately. Maybe I'll try the vice.
Yes, there was threadlock on the other bolt, which came out, and also on the boss where it screwed into the brazing/seatstay.
Both boss and bolt are steel: will heat help? Very difficult to get to the relevant parts, with the brake caliper arm around it/them.
Have taken the two front brakes off and the bolts are only 18mm (the 'normal' length). So the one giving the trouble has an extra 10mm of engagement (at least 20mm altogether). Perhaps not surprisingly the boss (about 8mm but wider dia) 'gave' first.
Status quo ante option is to just screw the boss back in and accept an aesthetically unpleasing rusty head (suggesting a father doesn't care for 'his' bikes.

 

[Cerdic]

Heat. Get a blowtorch on it....

 

[the snail]

You could screw the boss back with threadlock, and see if the bolt unscrews ?

 

[si_c]

Have you tried fitting an Allen key in the reverse, see if you can then unscrew it?

 

[Ian H]

If you can find a nut which fits the boss, saw through one side, screw it on to the boss, then clamp the nut in a vice. This should hold it firmly enough to remove the bolt.

 

[Ajax Bay]

Again, thanks all. I had a final go (after penetrating oil in for an hour one way and then an hour the other) with mole grips @Cycleops (before going for the vice) and, happy days, it budged.
The boss' thread's a bit damaged but careful screwing it back in, with threadlocker, got it back in its rightful place.
One wonders why the builder chose to use such a long bolt (27mm) on those two back ones whereas the front ones were the normal 18mm.
@FrankCrank there are flats on the boss, but they're hidden when the caliper is 'on' (and this also prevents effective use of heat, imo).
Thank you to @the snail @Ian H for those suggestions, to which I did not resort. @si_c I did try that but no purchase/effect.
Now for the wheels: why would Islabikes use plain steel spokes; not even galvanised? I have 40 (+2) x 180mm SS plain gauge spokes ready to lace.