If I recall correctly, Rent Redi even gives them the option to pay using a credit card. 

I have two banks that have done so as well as a credit card, and know of a couple of organizations that have as well.

But they were selling an expensive house in California and had horrible credit.

You would want to look for an owner that has a loan through a bank or credit union or one that has no mortgage. 

Quote from @Griffin Malcolm: Open an LLC and apply for a small business line of credit from Key Bank.

I would say credit monitoring

I think they used to go up to like 80-85% CLTV depending on credit score.

Not for a single airbnb like you are seeking (you and everyone else) but because if you start a BNB or a guest house or a hotel, you can get 40% of your investment back in the form of a tax CREDIT that you can sell for about 90 cents on the dollar. 

This is much lower than the costs associated with refinancing.Benefits of Mortgage RecastingLower Monthly Payments: The most immediate benefit is a reduced monthly payment, which can free up cash for other financial goals.Save on Interest: By reducing your principal balance, you’ll pay less interest over the life of the loan, potentially saving thousands of dollars.No Credit Check or Appraisal: Unlike refinancing, recasting doesn’t require a credit check, income verification, or home appraisal, making it a quicker and simpler process.Keep Your Current Interest Rate: If you have a favorable interest rate on your current loan, recasting allows you to maintain it while still lowering your payments.Drawbacks of Mortgage RecastingRequires a Lump Sum: Not everyone has the cash on hand to make a significant lump sum payment, which can make recasting inaccessible for some homeowners.Doesn’t Shorten Loan Term: While your monthly payments are lower, recasting doesn’t reduce the length of your loan.

It sounds like you're trying to do right by your other tenants while also being fair to this one, but it’s clear the situation can’t continue.First, it’s great that you’ve already put quiet hours in place and handed out notices—smart move!