Re: head bolt
Sorry,I can't think of any adhesive or filler that would hold up. I think you should take it somewhere else for a second opinion. I wouldn't think the head studs would be so close to the bore that significant damage would be done by tapping to the next size for a helicoil, unless they have to go up several sizes to fit the helicoil. If so, then maybe going up only ONE size would be feasible.
You would have to make sure that the larger clearance hole in the head wouldn't be a problem, and you may have to modify the head of the bolt(or the nut on the stud) to fit it in a recess in the head if there is one, but that might save you.