I know you said everything, but
I would first by a socket Hex key and a breaker bar and have a go after a couple days soaking in lubricant.
Something will move - it will either come out or break
If it breaks you will have to drill out the stud and re-tap,
That is what JWhiting did to mine several years ago.
There is room i think to drill out the studs in situ, but I would not trust myself to get it drilled straight
The only downside is that if you break the bolt at home you have an undrivable car but I would suggest you would get a better job taking the head off and getting it to a machine/engine shop
One you have the head in the machine shop there is no better time to get other things done.
Nick