Lulu's already given you some great advice, so I'll just answer re: the mortgage. The best option would be to get legal advice regarding that, as it is an asset that belongs to both you and DH. He would still have to pay you child support, you actually have to apply for child support in order to receive certain payments from Centrelink (sorry can't remember all details, been awhile).

And big hugs for you, I'm sure it's not an easy decision but hopefully will be the best one for you, your girls and your hubby as well. One of my fav. Dr Phil-isms that I think applies to this situation is "children would rather be from a broken home than in a broken home".

Hope everything works out for you