[mothered]

I'm thinking of editing the POP sent by the individual seller and only changing his personal credentials, leaving order number etc untouched. Would that work?

The order number/Invoice number Is what companies/manufacturers use to verify the POP Itself, so unless they delve Into the buyer's details, your plan will suffice.

Or would it be better to just make a fake POP from the same store?

If It's cross-checked, I don't need to explain what happens.