In short, it would negate any advantage to making any early payments, with this advantage to the lender balanced by providing a noticeably lower monthly payment than the $1548.67 to the borrower.

Partner with a lender.

personal finances and mortgage with local lender How did you add value to the deal?

Based on info provided, rate should be more in the mid 7s-low 8s likely, back of the napkin, with no lender discount. 

My question is that if a deal has a mortgage subject to an existiting loan and a private lender funds the purchase of the enry fee (wholesaler fee/agent fee/cash to seller/etc.)

What you need to do is hopefully go to the lender's website (that your relative was using to pay the mortgage) and just make payments to the bank. 

Do you have any existing insurance to compare it to or you've just been getting quotes on properties that you are considering purchasing?

Most lenders here on BP will do value add loans on 1-4 units.

Does resent comparable sales impact office values in same building at all or we only need to look at CAP rates and determine the value based on CAP rates alone?

Be up front with your lenders about your scope of work and who you intend to hire to get it done.