As far as the welding, well, if using a cutoff wheel and re-shaping the hex to fit a slightly smaller six-point socket doesn't work, do NOT cut a slot in it for a screwdriver bit, or if it's that tight it will peel the edges right off.
If no joy, find a nut that will fit onto the remains of the pan bolt.
Tight enough it will take some tightening with a big wrench to get it onto the remains.
Then, arc-weld the nut to the plug on the INSIDE of the nut to the plug.
Sometimes, the act of heating will expand the metal plug into the corrosion in the aluminium pan and when cool will allow easier removal. Once cool, and you're satisfied with the weld, remove the nut and plug with a box-end or socket on breaker bar. Do Not use an impact unless your are darn certain your weld is certifiable, or you'll just break the weld.