You can refi up to the HML principal (+ closing fees?)

If you do sell within the next three years, you'll probably come out even, as you added the additional loan costs to the principal you need to pay down and your house will may be underwater (assuming you bought at market and there isn't crazy appreciation where you bought your house). 

I am a principal commercial broker and also a commercial developer and I have plenty of commercial opportunities where I live.I do not invest in SFR.

Would you and should you somehow use that cash to pay down principal first and use your own salary to pay monthly mortgage payment? 

While principal paydown isn't usually going to come in to play in a 5 year period, you will also have more equity from the very beginning due to the value add.

If that number factors in maintenance, vacancy, capex reserves in addition to both principal/interest payments and the taxes and insurance I would definitely do the deal. $2580 of annual returns on a $0 investment is a hell of a return. 

When you know what someone wants and giving them what they want gets you what you want principals are a foolish waste of energy.

If I were to put the money in an investment such as a stock that I would expect to earn me say 6-8% (obv still risk to lose the principal), it is slightly different then my scenario where the investment such as the money for a refinance is gone right away and therefor would have to like you said "pay myself" back the principal first before anything beyond would be considered profit and therefore would obv have to also account for the time value of money since it would take even 1.5 years or so with a refi rate of $2800. 

Next, adding interest to an underlying loan, several issues, but your interest rate won't match the principal owing on the underlying mortgage this makes tax reporting and borrower disclosures difficult, beyond the financial scope of any small investor without a loan compliance department. 

You can get very reliable and accurate contact info for principals using LexisNexis or TLO.