One of mine came out with an air chisel which I bought especially to try and finish the job, it was not that useful due to the obstructions making the angle of attack obtuse., The other side was solid despite plus-gas over a month. Air chisel got nowhere maybe moved it 1mm, didn't want to heat too much due to it being aluminium, I tried it but it just doesn't hold the heat like steel.
In the end, I turned the head with a breaker bar until it sheared off, then then started to drill it out with bigger and bigger bits while squirting oil on until I was worried obout it bursting out.
After a bit, I took the nut off and put a greased washer on the threads and put the nut back and just tightened it and it started pulling what was left out little by little until the nut reached the plain bit at the end of the threads. After that it was out enough to use a hammer and an old 8mm drill bit.
Lathered the whole thing up with Waxoyl when replacing.