I recently stripped an E12 and was able to hammer an E10 onto the head. Ended up twisting the head off and drilling out the bolt but clearly the head was grabbed well enough.

Do you have side access where you can cold chisel an edge to try to turn the bolt? Not a very big bolt if it's not fused the torque should be under 40 ft·lb may likely be able to chisel turn it out

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)