Bit of an update to close this off.
Spoke to a different lbs as a last resort before packing bike off to Germany post my reentry at canyon. They seemed utterly confident they could fix it, no mention of warranty issues etc, but a couple of weeks lead as they’re super busy. So I wait with baited breath.
They phoned me today….and…..fixed! Just like that! So mildly embarrassing post my rant at canyon, but so relieved to get it sorted! Turns out the bolt was loose but not due to the threads being destroyed, just due to it not being tightened up! I don’t know how to do an embarrassed emoticon, but if I did, it’d be here.
In my defence, you can’t access the bolt without removing the cranks, so I could only take the word of my lbs on the damaged thread.
So anyway, the upshot is my bike is fixed so I’m back out riding again!