Good effort so far.
1. A flap wheel is not for that. Use a genuine hard wheel made for metal.
2. Cut slots in the head if that's all you can reach. Then run the wheel back and forth or knock off the head with a chisel.
3. Drilled a small hole? Drill a 1/4 inch diameter hole by 1/4 deep. Center punch, drill 1/8 x 1/8 deep first, if you want.
4. Are you saying that the cross member is not loose? If cross member is tight or if it looks like having the cross member involved will create too many problems, maybe you can push the bracket up and in tight, hold it with the new side bolts and drill two extra holes for the bottom new bolts. Just a thought before you realize you have to take the cross member off or there is no access to de-rivet.
5. You can't damage the bracket by just kissing it with the grinder. A few shallow gouges will not hurt it.
6. Don't Mickey Mouse with this... cut, grind, man handle, chisel, punch and blast. BFH, kill and maim.
Not Dremel/flap disc/die grinder magnitude. 4 1/2" angle grinder 3/16-1/4 thick wheel magnitude. Sparks everywhere ...yeah!!!
