OP, I sympathise. With the benefit of hindsight, you should have put boundaries and mutual expectations on paper, but grief etc got in the way.
If you have kept old utility bills, I would look back at those pre-extension and look at energy/water use in terms of units over a year. Then look at post-extension. The only real difference in that is your DM's presence. Then calculate the current cost of those units, which should then be her contribution. Similarly with council tax. If the extension means you have gone up by a band, she should pay the difference. She should be paying a third of the insurance, broadband and TV licence.
Then I would set up some ground rules. Now the DC are older, presumably they are in school/nursery more regular hours. In return for lifts, she will agree to do the emergency childcare. No childcare = no lifts. I would also ask if she would like to eat with you on regular occasions. In my family we have an unwritten rule that the chef/cook doesn't have to wash up. If mum wants to cook the family meal once or twice a week, it saves you a job. If she doesn't, you haven't lost anything as you'd be doing it anyway.
I have a similar situation in that a parent has part of my property for their exclusive use, however, they are not resident here permanently. They looked at the cost of renting a similar size place in this area when deciding how much to contribute. Their utility costs are separately metered but within our bills so we can see how much their part costs. The amount is reviewed annually. We do all the external maintenance but they help with gardening/minor repairs when here and are considerably older than your DM.
I think you need a child-free discussion with your DM and DH about the way forward. It won't be easy but will hopefully lessen the resentment.