is the spike jump while your welding or not ?
or is it just where only the trigger is being pushed
Again, i know nothing of welder design, but im sure id be tempted to jumper the whole burnt section with a decent bit of wire, and tentatively turn on the device.
Im not saying run like that, but id at least want to gather some data by trying.
When I was prototype/repair wireman years a go we had a particular PCB that used to come in for rework where doubled up the tracks by fitting some heavier solid wire linking the same connections as the tracks weren't able to cope so I don't see running like that would too much of an issue.
dont rely on it get a replacement as soon as you can always replace with like for like or better
The diode/zener thing? Any reason it shouldn't be ok? It's higher rated than the original and only ever sees half of its cut off voltage.