Loan is an FHA loan at 4.25% and I am of course still paying that PMI. 

He pays rent on time.

Hypothetically, if I took over someones existing mortgage subject to and owned the property cause I have the title now, would I be able to get a home equity line of credit or equity loan on that property if it had a good amount of equity?

Debt is not a bad thing, and you can always use the additional cash flow from a property to pay it down quickly if that is part of your goal. 

If they refuse to pay, contest the charges with the credit card company (assuming they paid with a card).

Now you need to figure out how to find deals and pay for them.

Hello,I'm reading Avery Carl's book "Short-Term Rental Long-Term Wealth" and she mentions setting up a designated business account and making housekeepers fill out a W9 and paying them through direct deposit or an online invoice payment platform.

The first thing a scam artist will do is build a fancy looking website and pay for some reviews. 

You may pay $10,000 in interest but you only get back $2,000 back in tax savings. $2,000 is just a random number, the exact number depends on your overall tax situation.Building an ADU if you plan to rent it is business interest, not personal interest.

If you lived in the property for 2 of the last 5 years then you do not pay capital gains tax when you sell.