cheers have seen induction heaters they are very useful but in this application they will case harden the stud and not directly heat the cylinder head.
Err no.............. Case hardening takes several cycles of heating and cooling, plus the carbon content of the steel is a major factor, these are low ish carbon content, basically it will do very little. The use of an induction coil is a good idea, they heat both the stud (to break the corrosion) and the head, they've never failed to work for me if they can be loosened.
I am mechanical engineer to trade so no issue there but looking at no way in hell can drill out in place access fouls direct line if sight ie drill perfectly flat. Main issue is lack of space and workshop tooling and equipment ie oxyacetylene.
As said before depending on your skill and tools you should be able to drill them out. I've done, well I've lost count, never as yet had to take the engine out. Remove the rad at most remove the front end at the head stock at worst. Drill out the broken off studs, often cheap easyouts don't work. You need to drill out the whole stud including the thread, retap, make up or have made a threaded stud the size of the hole now in the head. Screw a length into each hole, using bearing fit, redrill/tap the plugs, fit new studs, make sure you cooper slip the threads. Job done.
baffles my why use mild steel in alloy head with tiny m6 threads
Its cheap.
Yes you can run the engine all day long without the headers, but why? I wouldn't want to be you neighbours (if you've got any) either