Hi Claire, First, I know it's tough, but you have to accept that even if your Mil loves you like a daughter, her allegiance is always going to be with her son. Try not to take any of what she's doing personally as she's just trying to navigate how to love her son even while he's making bad choices. Let your H have her on his team and concentrate on getting support from elsewhere right now.
Second, about the babysitter thing. If he was willing to change his work arrangements to work from your house, there is no reason you can't also be flexible on the schedule and offer to come home earlier in the future. The babysitter messed up his plans (to go in to work) so it's not a weakness or flaw to also pitch in and change your plans if you are able to help out.
BD: Aug 2012 Separated since May 2013 S born Aug 2013 Aug 2013 H agrees to consider 'baby steps toward working things out' H is/was actively seeing someone?