In the hot market we have been experiencing, the only good reason for having no offers is that:
The asking price is too high for a house of that type,size and condition in THAT neighborhood.
YES, ALL closing costs if sold through a broker should be deducted out, as that could very well be necessary. Also Title and closing fees, municipal code compliance, municipal letters costs, well and septic tests, prorations, mortage payoffs, payoffs of any liens or judgements, recording fees for releases, homeowners association transfer fees, state or county transfer taxes or revenue stamp fees, etc.
A qualified mortgage appraiser who is active in YOUR neighborhood and knows the nuance of what the market bears and what makes any one block pricier than another can provide an appraisal to use for value. Be aware that appraisal is an art and not an exact science. A certain degree of deviation between appraisal values is presumed.
I also STRONGLY recommend that a title update or letter report be done PRIOR to buyout so that any unknown liens or judgements that have attached to the other parties interest are dealt with. Also, refi the mortgage and get the other parties name OFF that debt.
I do not know how to answer the question about what date to use for valuation. Likely the value is similar. It may depend on whether one person alone has been making all the payments since a certain date. I would think that would be negotiated.