Interesting...I agree you should waste too much time on it or respond.
Optimistically, she might be apologizing for the affair...and hopefully that means its in the past. Or, she's trying to get under your skin because it isn't going well. But, I wouldn't analyze it more than you have.
Are you going to show the note to H? I probably would if it was me...I'd want to see his reaction and also show him what she said. I'd be nice about it and simply show it to him...but maybe that's too hard to do and if you are making progress with him maybe it isn't worth any risk. I just think putting it out in the open might help.
Hang in there...you are creating your own path and one where you won't have any regrets over not trying!
M 44, H 46 D11, D9, D5 Married 12 years PA confirmed 9/2011 I filed 3/2012 H moved out 7/2012