I would suggest the prefix "vicari-" from the latin for "substitute" or "deputy." I know this from biogeography, where it is used to describe cases in which the species (or higher taxa) living in different areas are phylogenetic substitutes for one another. In terms of how to create a neologism here are some options:
1) "Vicarispouse" - but I don't like the mixing of the latin prefix with the English "Spouse." (Vucarihusband/vicariwife are even worse)
2) "Vicarisponsus" (M) "Vicarisponsa" (F) All latin - but hardly likely to catch on
3) "Vicariant" A valid English word already, but perhaps it could be adopted for this use?
Maybe I'm missing something, but what's wrong with referring to her simply as his wife? If for some reason you must explain his relationship to you, he is your ex-husband. And if the person you're talking to already knows you have children, it would hardly seem necessary to say something as strange as "my children's stepmother."
Possible term for new spouse of your ex-spouse.
In the Theater an understudy is someone that is prepared to take over a role when the original actor can not appear for any reason. Thus the replacement spouse could be called an "underspouse" by analogy. It also gives a hint of high ground to the original spouse without being blatant. there are many movies based on intrigue between the original actor and the understudy, which again often would reflect reality.
Hmmmm, David, I don't know -- sounds vaguely naughty to me! 🙂