In this type of situation, they want low interest rate with low down payment also.

I think it was for down payment and closing costs.

If the other two parties want to flip a house maybe they can do just that as partners and then sell you the house which you can buy with a primary home loan with low down payment.

What would stop him from just not coming back to finish once he gets the last payment from you?

And offset the previous FHA loan payment showing that you're making the payments for it so it lowers his DTI.

Does a DSCR loan require 20-25% down payment?

When revenue is low in the first couple of year, so is the interest payment.  

If you have extra cash at the end of the month, then do double up payments on the mortgages so the principle is reduced and this will increase your heloc room.

I financed this property using down payment funds from 403B and a local commercial bank. 15% down from personal funds. 85% down from community commercial bank. 3yr balloon, 20yr amortization, 7% interest rate.

To qualify for a loan, focus on having a credit score of at least 620 (or 580 for FHA loans), keeping your total monthly debt under 43% of your income, and saving enough for a down payment and closing costs—about 5-10% of the home's price in total.