My situation is that my STR averages less than 7 days, I have material non-passive participation, so that I qualify for the "loophole". 

You will be be responsible for the transfer taxes equal to the assessed value, plus deed preparation fees and if there is already a loan, that adds another layer of complexity.

I think they mainly help people determine if unpermitted additions can (likely) be made legal or not, but seems like they could be useful in your situation as well.  

A key component to cash flow will be loan terms..you could consider trying to incorporate a seller concession towards an interest rate buy down or costs to help improve cash on cash returns.

The intent is torefinance them once I am done with the rehabs and I want to stay clear of theseasoning restrictions often seen on conventional loans.

As well as moving from higher cap ex exposure to lower cap ex exposure.Cash flow may go down but when the other components of the IRR go up ( appreciation, depreciation, and amortization of the loan) your IRR goes to the moon and more than offsets any drop in cash flowJust make sure you stress test and can withstand any drop in cash flow.

Here are a few ideas:- HELOC: depending on how much equity you have in your property, you may be able to apply for and borrow against that equity- DSCR: BP now has a find a lender link and I'm happy to recommend mine as well if you'd like; there are lenders who do loans based on the future rental value of the property- Borrow from your 401k: typically you're able to borrow up to $50k and if you have multiple 401ks, borrow from multiplesHopefully this gives you some ideas of other options to bring in some cash for your investment!

Let's say I have 500k down payment and borrow 500k, and the monthly mortgage payment is 5800, and out of that, 4800 is mortgage interest, and I pay 2500 annually for insurance and 10000 for property tax, could anyone ...

Could get a personal loan beforehand to help cover the difference.  

There are some very high LTV investor loan programs.