Ryan,
I, like others, have been highly anticipating the outcome of this deal.
If you made your best offer, the one you're comfortable with at $65k, then I would stand by that but leave the door open with the seller so she can return to you without loss of face. I would stand by the 70% of ARV minus rehab costs. It seems like you are negotiating with your better judgment in thinking about paying more. Some guru or another says you make your money at purchase. Wise words.
What's the market like in your area for small (<1000 sq ft) houses without garages that have small 3rd bedroom additions [that may be funky or diy]? If you're really not sure, be conservative.
I also would be leery about partnering with the homeowner who couldn't finish the house in the first place. If she harbors any issues/resentment about the arrangement, there's potential for that to go sour. I would personally buy or not buy in this situation.
Maybe I'm being super conservative, but it's in my nature to be cautious. But I'm also optimistic about your finding even better deals--you just have to get them in your pipeline. And then you'll get a return call from this seller asking if your offer is still good. Good luck! I'm rooting for you.
--Michael