This is basically an unsecured personal loan, like a credit card.

Some lenders will let you both be on the loan and use the Higher of the credit scores between you and your credit partner. 

Focus on getting your finances back on track first—start saving and work on building your credit score.

If you want to do some flips or BRRRRs then you could consider taking out a Line of Credit against the three properties so you can use the cash with interest only payments for a short term project and then pay it back and then use it again (Lines of Credit are revolving - meaning you get to keep using the money).

I'm not sure what your local options might look like with a small community bank or a credit union. 

Since living here I’ve put in a new HVAC in both units and kitchen into one unit, I’ve max out my credit card, and have had my parents help me.

Look carefully at the credit card transfer checks.

Working with local credit union to refinance at lower than market rates

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

If you’re running a MTR more like a STR, then you can decide to charge the full boat as agreed or, if you desire (possibly to obtain a good review) find a substitute renter for the period in question and credit the original renter.If, like me you operate a MTR more like a LTR just for a shorter period of time, you probably use a specific rental form provided by either the state real estate commission of the local NAR affiliate.