Rental age of clients, past due length, how many are theirs or non paying.  2.  

If you are paying for all of the utilities it may come out to $200 - $250 per month during winter for Electric and Gas depending on size of unit age and efficiency of furnace and other factors.

But in this day and age that is the wrong thing to focus on. 

Will banks refi at 75-80% without requiring income verification or bank statements?

Since most of them are under age 18 it doesn't concern me that much, but all adults should definitely be run through your screening.

So, at my age, with 3 school aged children, would you pay off the mortgage and invest the rest in a portfolio, real estate, or IRA (125k).. or leave +/-50k in the bank and invest the rest?

No one can really answer this question without knowing more about your life situation (goals, savings, age, income, etc.), but I fully agree with @Joe Villeneuve. 120K equity to get you $100 a month CF plus whatever minimal equity your gaining doesn't sound good at all to me.

Age of accounts, history, and balances more important.

If they are VERY professional, they will have their processes in writing as verification that policies are enforced equally and fairly by their entire staff.6.

You are describing the perfect example of a situation needing a "DSCR" Loan - made exactly for this situation, meant for turnkey (fully renovated and rented) properties and qualification based on the property, not the income (no DTI, no income verification needed).