Those other problems like asbestos, etc. mean that I need to have money in reserve to deal with violations and other issues.

The bad part is that if that property that you buy is being professionally managed by another firm when that renter goes to that firm to reserve it for this year, that firm is going to say "that property has been sold, here's another one that's similar".  

However, it is true that they want to see some kind of reserves, usually with the $15k minimum you mentioned.  

The lending requirements for a refi are the same as a purchase so you will still be held to the normal credit / DTI / Reserves requirements.

How much do you have in reserves after down payment and closing?

I see both the sub too and sandwich lease as being an advanced scheme that should only be done by folks that have substantial reserves and credit etc.. 

The topic is complex with possible discussion of fiat money and reserve currency and the role of the Fed and inflation, consumer debt, etc. that could fill a book.

Personally - my current househack allows us to profit $1k per month (not including reserves,maintenance)Our first househack we lived for free and made about $200 per month while living there.

If you had cash reserves for a 25% down payment for your next income property, the bank would help you own it, the tenants would pay the upkeep and mortgage.

It's listed on AirBnb and I require that they go through it for reservations.