I'm not certain of my debt to income ratio at the moment, but if I try to mortgage out a property to purchase another, does the rental income I anticipate getting from the new purchase factor into the debt to income calculation?

Whether you're aiming for short-term gains or long-term passive income, the choice between flipping and renting can significantly impact your financial goals.

especially without being able to increase w2 income very fast?

I don't have a W2 and all I have is the first duplex income and some photography income from my photo business.

You’re able to defer taxes by accelerating the depreciation of specific assets which helps reduce your taxable income, thus deferring your tax liability into future years.

When looking for a DSCR lender for your short-term rental, ask how they can underwrite short-term rental income.

All or some expenses run through the PM and offset the income?

It depends on your income / DTI and also what kind of loan you are going for.

If you are looking at a hard money loan, most will only care about the equity in the deal, and your experience in the last 36 months If you are looking for conventional, full doc financing, they won't care about your experience, but will look into your personal income, credit and assets. 

My understanding is comparison sales approach or income approach.Comp Sales approach is getting comps from area and then carving carving 40% +/- off that price.