i think if you leave negative feedback, and then they do the right thing, you and the seller can mutually retract the feedback, I know that used to be an option.
I have done this in the past with a seller - I left neg feedback for her and she left neg feedback for me. Once it was all resolved we agreed to retract the feedbacks. Not sure if it is still an option though.

ETA - another thought, usually when I am not 100% happy I leave neutral fb, that way I can still voice my issues even if things were resolved eventually etc.
We did this for an item DP bought... it wasn't enough to warrant neg feedback but we wanted to raise concerns we had - even if everything was to be resolved they were still our concerns.

It certainly is a lot of time taken to respond to you and I can understand your frustrations