Quote from @Pete Tarin: How does one go about assuming someone's mortgage with a better interest rate when purchasing a property? 

Most lenders use FICO credit mortgage credit scores.

The loan doesn't decide its lien position the order in which voluntary liens (mortgages) are files determine that. 2.

@Armando Carrera as others said above that’s mortgage fraud and not at all what an FHA loan is for.

Anticipation of these cuts initially moved mortgage rates downwards to their lowest in nearly 20 months.

So, if the value of your home is $775,000, then 80% = $620,000.That's first.That $620,000 represents the MAXIMUM mortgage amount. 

Then use the extra roommate rent income to pay down your mortgage and look to refi to a lower payment in the future (3-5 years) - so it cashflow when you move out.You could also choose the smaller unit and STR the other unit to generate more cash to pay down your mortgage and refi, so it cashflows when you move out.Bottom line: you'll have to get creative to solve your cashflow challenge!

Numbers... not words.The property tax is $4000/year and I guess I can be thankful that I paid cash for this house because if I had a mortgage I would be paying significantly out of pocket every year since I have owned this home. 

House hacking lets you live in one unit and rent out the others, which can help cover your mortgage and generate some extra income.

A cash-out refinance would mean taking on an entirely new mortgage, which may not be necessary at this time.