I have owned this property for over 20 years and have a low balance on the mortgage.

Then a Google Doc to track my monthly cashflow and balance sheet.

Ask the loan officer if there is somewhere else to list all debts and their outstanding balances. 

I’ve run the numbers ... we will pay twice as much as the loan balance in interest and taxable income if we do that rout vs just paying it off in 20 years. 

She is also requesting that we waive her balance.

For you to carry a balance and pay them higher than market rates on a loan that you don’t need to pay back as fast as you should.

With an 8%+ rate you can ask for a 20 year no balloon as the seller may be able to sell the note at a reasonable discount once it seasons.My initial offer would be something like 20% down, owner finance balance at 6.5% interest amortized over 20 years with a 10 year balloon.

@Danielle ScottValue $350k, what is loan unpaid balance?

I have a balance of roughly $11,000 on one card used for work.Additional details: I came across the newest warehouse (investment) property earlier this year and used most of my free cash for that purchase (June 2018). 

You're basically computing the APR and computed the same way as the IRR (which might be where confusion lies) in that you have a present value (your cost), a future value (the balance owed) and a note payment and you solve for the interest rate.