Your DH is mired in FOG (fear, obligation, guilt) when it comes to his mother now and this state as well as his own inertia is hurting his own family unit.
His parents have always lived beyond their means and expect their adult children to keep bailing them out. Her late husband was her willing enabler and himself a weak man also.
TBH your DH giving her your family unit's money is basically chucking good after bad; its not going to make the problems go away permanently. Its basically putting off for now the inevitable financial collapse. Presumably she has no funds to maintain the upkeep of this house (I believe from the other thread she has also had a gardener paid for).
He has to change that habits of a lifetime and not give into such emotional blackmail. It is bloody hard for him to do that but he has to say enough; he could lose you over this issue all too easily.
I presume you come from a nice emotionally healthy family yourself and that is why you have stated that you would not want to stop her seeing the children. However, you are not and never have dealt with someone like his mother before now most likely so the rulebook regarding familial relations goes out the window.
I was wondering why you feel at all sorry for this woman anyway; this is after all someone who told your DD that she is only interested in children up to the age of 3 or 4. She needs to be now kept well away from your children (look at how she has damaged your DH; similar will happen to your children too). You would not have tolerated any of this from a friend, family are no different.
You all need to stay well away from his mother and your own relationship with her should have ended long before now.