It sounds like the best way to remove it would be trying to make additional payments on my mortgage to get to the 80% mark and then refinance if rates justify a lower monthly payment.

What would be my best bet as to start up money or money up front ?

Remember this is only for Fannie/Freddie loans, and they allow up to 10 the first 4 are just easier and can be cash out refinances.

If I'm off on my rehab costs or my ARV, all bets are off.

Most times they weasel is when they note the issue in their commitment, like of access or will not cover location of property line if no survey done etc.If you ever try to refinance some of your money out or use it as collateral you will have to buy it anyway.

My plan is to purchase the house subject to the existing financing (I take title and pay sellers loan payments), rehab the house with cash, then refinance and potentially cash out my rehab money if the numbers are good enough.

Is this deal similar to your first four properties or have you refined what you're looking for?

While your imediate area may be tought to cash flow, I bet within an hours drive there are neighbrhoods that will make great rentals.

Your best bet will be smaller local banks.

That's been done but is very rare.I'd suggest you walk away first, but if you must, don't do an installment contract as you won't have enough room to refinance or finance the deal without cash injected, if that's at hand, just buy it.You can cover payments with a lease.