If there is an answer to your question that originally isn't acceptable but later becomes very acceptable after discussion through comments should you ask the person to edit the original answer before accepting?
It's always best to have all the relevant information included in the answer. Comments are easy to miss and hard to follow (especially in a longer "thread"). One of the things that's great about Stack Overflow is that people can find answers to their question without having to sift through a bunch of irrelevant stuff, and bringing useful information out of comments and into answers helps with that.
You can either ask the user to edit the information into the answer, or you can propose an edit yourself. Once you have 2000 reputation, you'll be able to just edit without approval, but until then, you can submit a suggested edit. I recommend leaving a clear edit summary to explain that you're including additional information from comments so that reviewers don't think you're some crazy person radically changing the answer. Which they still might, but if that happened, asking the OP to edit would be my next step.
If you simply can't get the edits to go through and the OP isn't responding, Brad Christie brings up a good option - post an answer of your own with all the relevant information. I would credit the person who helped you the most in that case in that answer as well, just to be polite and respectful of their time.
There are a few solutions that I see:
First, would be to ask the user would posted the answer to edit their answer to include the relevant details from the comments, once that is done then you can accept the answer.
Second, if the user does not edit the answer, then you can attempt to perform the edit yourself.
Third, if you don't have enough rep to edit without going through the review queue and your revision does not get accepted, then you can use all of the information from the comments to post a final solution to your problem. Then at least your question will have been answered.
If the user's original answer wasn't the solution but they did (through comments and discussion) bring you to the conclusion
If they don't modify their answer though, it's probably safe to create a new answer and post what got you through the problem and accept it. This will close out the question affirmatively and define the solution for future reference.