More or less what I had said , but its way easier to open it up and do it properly ,, Im a welder not a gynaecologist.Could you carefully open the hole out until it was large enough to accommodate a full nut of the correct size and then using a bolt to hold it and protect the threads weld it into place from the outside? The natural gaps between the circular hole and the hexagon sides of the nut should allow for plenty of penetration. Grind the welds smooth and check. Best to cleanthe plating off the nut before hand.
No , def not stainless , not if you ever want it out again , too many different colours , like boat prop shafts ,, the fewer different colours the better. and I may be quoted on that one.If theres no other way then go oversize and maybe use a stainless insert?
Not sure I like it though