I think we need to hear both sides of the story.
In my opinion, the chain fell inside while riding in a warped state, locking the wheel and causing it to fall.
Rear-end collisions often cause axle bending, so it's likely that the axle bent after the accident.
You don't have to use exactly the same spokes. You can cut and process them appropriately for use without any problems.
The manual says something about replacing it, but as long as it doesn't get longer than the specifications, fold in half, or anything like that, you can adjust it accordingly and set the tension properly.
Most shops don't do this, but the mechanic who fixed it seems to be responsible and takes pride in his work.
Anyway, the point is
Whether you just took out the rear wheel and left it at the shop, or left the whole bike.
If you left the whole bike, did you let them know about the rear-end collision? That's the key.
-Summary-
0. Since you rode over 100km after the overhaul, there's no problem with the initial setting > Mechanic is not at fault.
1. If you left the bike with the broken spokes,
> If you didn't let them know about the rear-end collision and didn't request a full inspection, then > You are at fault.
> If you didn't let them know about the rear-end collision, but the mechanic mentioned the axle problem, then > You are at fault.
> If you told them about the rear-end collision, but the mechanic didn't point out the problem, then > Mechanic is at fault.
2. If you only left the rear wheel for repair,
> The mechanic wouldn't know the condition of the whole bike > Not at fault.
