Ok, the deal with "dual agency" is a slippery slope and as said above, many don't know what they don't know. The first priority in a "conflict of interest" situation is to "not be involved with the conflict" in other words and agent that know about this subject will steer clear of ANY discussion on the point. After all its his license that's at risk. Rather than go into more detail about that subject it would serve better if I made a suggestion on how to negotiate a little better.
The experience of the sellers agent is showing in that you came at them with a super low-ball offer (most deals never get to 5% off asking). The standard counter to a low offer is a high counter. That said, you've only got 1 more chance to get it right. Split the difference or go as high as you can with a full accounting of the details needed to fix the property. Suggest they do some or all of the work or you'll find another house to buy. Sometimes it works, but often it does not. Buying wholesale is a hit and miss proposition but if you get 1 in 20, your still a winner, and as we say in Las Vegas "chicken dinner"!