He paid it with his money, but was reimbursed with campaign money because explicitly billed it as a campaign expense.
"Trump did not pay Cohen with campaign money."
He did - it becomes campaign money when you pay a campaign expense.
"So what FEC paperwork is required again?"
Any payment over $200 requires disclosure. It wasn't disclosed and that is one of the crimes Cohen is going to jail for.
You skipped the part where we both agreed NDAs are not campaign expenses.
Cohen did NOT bill as campaign expenses either. He billed it as attorney services rendered.
Trump paying an attorney with money from the Trump Organization for normal attorney services (and NDAs certainly fall within that description) are, once again, not campaign expenses.