What is a fair offer to make to that GC, and what kind of deal should I not accept?

Make enough offers and often times you'll suprise yourself what some ppl will accept some times. 

Even a contractor with 30+ years of experience only knows what they know - so, they may not know the most cost-effective solution:(5) Political trends that may lead to more controls on landlords - being forced to accept S8, not being able to deny for evictions, rent control, etc.

It was suficient/accepted by the lender that granted the mortgage for the duplex, so I did not anticipate running into this issue trying to get the heloc.

However, the seller decided to hold off on accepting my offer since the property had only been on the market for 7 days and they wanted to entertain other offers.This experience made me realize the importance of paying attention to the number of days a property has been on the market.

Are you saying DMM is acceptable because other methods CAN fail too?

Tenant can still pay rent, late fees, and other penalties in full and get back into my good graces, but I do not waive any fees, accept payments, etc.

We didn’t accept the payment and requested proper payment.

They are the professional and should have told me what products were acceptable and what were not so they need to accept at least some responsibility.

But the guru/mentors are happy to accept everyone’s money - so they must sell the proposition that ANYONE who buys in to their program will be successful if they just “apply themselves”.