The short answer to your question is yes... for now, it is a bad idea. And look, I am all for second chances - heck, if I did not believe that people can change, I would not be doing this for a living. BUT, I am assuming that there have been some extremely bad times in the past - culminating with his abandoning you not too very long ago. You are still reeling from that blow; and when you combine your emotional vulnerability right now with his capacity to manipulate (if he truly is NPD, he is a master manipulator) you have a scenario whereby you are too close to the matter to get an accurate "read."
I like that you are using this forum, but I urge you to get (AND USE) the feedback from those close to you (friends, family). They will be more inclined to see reality for what it is because they have not been married to the master manipulator. My advice for the short term will be to go ahead with everything as planned. If you decide to back off with the divorce, at the very least maintain the boundary with a legal separation. Only when he is placed under stress by you OR is frustrated will you know if he has changed. An NPD does not have a tremendous ability to tolerate frustration or boundaries... so he should show his spots when that happens.
But protect yourself for now. Take a breath and reconsider once the dust has had more time to settle. If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answer. Thanks-
I like your plan, and your rationale. You seem remarkably intact emotionally for having been with an NPD for so long! With your continued boundaries, he will grow frustrated that he is not able to manipulate/control you... this will force his hand at which point he will either (a) spin further out of control possibly become violent, or at the very least begin a barrage of demeaning, disparaging tirades toward you (or whatever his proclivity was while you were together and he wanted to dramatically make his point) OR (b) begin the change process and attempt to reconcile your differences WHILE not getting his immediate desires met.
If in the face of frustrating circumstances, he appears contrite and willing to allow you your space and independence, then I would give you the green light to move ahead. But I think your instincts are really pretty good for now!