Re:
Was just writing this and the previous post anticipated option 1 below....
Regret that there’s a bit of a clue on the frame sticker adjacent to the mount - ie ‘heat treated’. It’s difficult to work on some alloy frames if they are a post -fabrication heat-treated frame. I have pushed it quite a lot (retracking rear ends for example, which is a quite brutal affair with jacks and scaffolding poles) and theoretically it is totally unadvisable. But so far I have got away with it. You may too, but the warning is that the temper of the tubing changes with welding, and needs heat treating again to restore the ductility of the frame.
Ok....options....
I would check out whether you could file off the mount and use a banded front mech. Shimano do endless-band versions, so they could cope with having a bit of the base of the mount (1mm or so) still proud of the tube. That would preserve the qualities of the frame. You need to check whether the mech can be mounted in the right position relative to the cage of your existing mech. But the endless band ones have got quite a lot of throw adjustment. You’d need to cut the mount off with a hacksaw and then file very carefully.
Second option is to risk welding, ask a frame repairer - and there are some very very good ones who work with alloy - to fabricate a new mount and then weld it to the remains of the existing one. He or she will evaluate whether they can weld to the weld - which can be a problem. The fortunate thing is that this part of the frame is not heavily torsionally stressed, so a loss of ductility may not be an issue. A frame repairer would be able to judge this.
Third option is a variant of the first, which is to file off the old bracket but get an engineer to make up a new bracket in two halves, which can be bolted around the seat tube. This would consistent of two identical pieces, bolted on the opposite side of the mech, and with the mech bolt holding the mech side of the bracket together. This is quite a simple bit of design and build - essentially a thinnish flat plate with a slot in it, bent into a radius the same as the outer of the seattube. Any jobbing engineer could fabricate it. I have made such things myself - do it in paper or cardboard first, then in sheet aluminium.
I am not sure about how much each would cost - the first you could do entirely by yourself, as long as the mech would be in the right alignment, which you could comprehensively check before you do anything.
You’d need to get estimates for the second and third options.
