Sorry, it seems like my moderated posts are showing up timestamped on when I hit submit rather than when they were approved, unfortunately.

It sounds like you need some serious boundaries with your H and the kids in relation to OW. If verbal boundaries aren't working, then something written might help. If you think he wouldn't fly off the handle, maybe you could approach him, but, given how he doesn't seem to grasp the gravity of the situation with your S and the OW, you may need to involve a L. It sounded like the L fee you spoke of was just for filing the D. It may be less for drafting a separation agreement (although it may involve a few iterations if your H becomes testy about working it out). Have you tried finding a different L through the local bar association? They may be able to assist finding an L who does pro bono matters or does work on a sliding scale if you can't afford it. Not saying you definitely need one, just something to consider.