Try plenty of releasing agent,leave for a while and then mole grips,worth a go.also work the bolt in small amounts tighter and loose in small amounts this will help to break it loose.
Hi .... I had a similar scenario with one of the Allen bolts that hold the metal strip in place which secures the top of the front bumper in the engine bay. Tried a few things without success.
Then I saw a video on Youtube ..... the guy used a chisel to whack a small notch in the head of the bolt. Once you have a notch, angle the chisel to rotate the head anti-clockwise with sharp taps (I used a heavy head hammer). Worked a dream for me.
Thest might be worth a try on those bolts, hammer on an undersize one and crack them open, Mazza has had success using these on his Bognor lean to project thread over on zroadster.org.
https://www.toolstation.com/irwin-bolt-grip-nut-remover-set/p63045
Other options on ebay as well https://www.ebay.co.uk/p/23032586981?iid=124566908240