Dogtired, perhaps you'd be interested in the thoughts of someone in your present dp's situation...?
I am actually relived that there are men out there who can see a problem with exw and mean to address it.
I met and married a man in similar circumstances, and can only encourage you to follow the lengthy and detailed advice given by previous poster, gigglestar.
I wasn't ow, btw, but was still dragged into the post divorce battle by exw ("don't spend money on that woman and her son that you could be spending on your children" OR "you are spending more time with that woman's son then with your own").
Dh had practically no spending money at all although he earned pretty good wages as he was paying maintenance (already classed "luxury level" in this country ,I.e. Europe but not UK) + school fees for private school for both dc + any extra exw could think to throw in (resorting also to extorting money from inlaws for any number of after school activities she could think of).
Btw, exw earns above average income, like dh.
Then there was the blackmailing I.e. Despite money rolling in, dh was kept on tenterhooks about visitation rights (although blooming obvious to all and sundry that he was just about still primary caregiver, having them 50:50 to look after, the rest was mostly her parents and aunt).
Needless to say, I was aghast at the spineless attitude to exw being displayed, I was giving lodging and contributing to food, bills etc 50%, eventhough there was only my ds and me and three of them (eow and twice a week for full dinners I was cooking).
Was I resentful? Hell yes!!!
Some of the above conditions have changed, only because dh finally listened to me, his parents, his friends, his former inlaws (!) and now also the couple counseler we've been seeing for nearly a year (and after 4 years of this nonsense I'm still not sure if the changes are enough to keep me in this relationship).
Please don't think its just the money, I have a good wage, managed fine on my own as lone parent for 8 years and exw has an excellent salary herself, so no, his children's lifestyle hasn't changed one iota.
It's the unnecessary pandering to someone's whims that get me.
Interestingly, while she is still trying to squeeze money out of dh, she has a cocklodger of her own, literally a toyboy (about 8 years younger) with no discernible income. Is dh also funding him? Probably, yes.