Are you looking to continue doing up the bolt or replacing it? I'd assume the bolt is faulty and needs replacing.
I would also still MIG that rather than TIG because of the instant heat/weld that the MIG gives. The TIG is slower process even if it can be done at a cooler temperature, but you'll probably get a lot of heat sinking into the alloy head which may cause additional problems. For the spatter, just cover everything up.

