Fix pinhole leak on shower mixing valve
You have damaged the valve and it will need to be replaced.
Judging from the photos the &quot;bonnet nut&quot; Is not a nut and is not designed to be removable by turning it with a wrench. It has three copper pipes pressed or soldered into the back side so it can not be turned of as if it was one pipe with a threaded connection.
I believe any hack &quot;repair&quot; you do will not be sufficient and will fail in time and since it will be out of sight in a wall you will not know if it fails until it causes a very large drain in you bank account. I.E. Damage that will require significant work to replace and mitigate water damage and possible mold.