It varies a lot depending on install method, regional area, heck even a specific properties weather exposure. 

If that is your intention the slow flip method is not for you.

.- My Resources / Constraints: I'm in a by no means unique, but perhaps less common situation for those getting started; I'm a somewhat senior software engineer at a tech megacorp and should be able to save a couple of down payments per year for lower priced houses (100-200 in my area), or perhaps one medium priced house (300-500).

After about a month of no response on FF I switched my method to LTR.

I understand that if I rent this ADU out, then I can have a tax write off on the mortgage payment and insurance payment for the the ADU's sqft portion.

Then you can talk to the mortgage company or payment processor yourself. 

@Tom Stevensonyes, when I BRRRR I typically use my own cash for the down payment, and a hard money loan for the balance. 

That allows the payment to be low, manageable and easy to cash flow when you transition it into a rental.When you move out you will have more money in the bank, stronger assets, and be cash fluid in case you need to make an offer on more investment properties.

If rates drop in the future, refinancing again could lower your payment, but it’s good to make sure the numbers work now just in case.

Before you ask, No, they will not make any back payments.