Lean on friends and family where you can to support you for the next week or so, and channel your energy into doing your best at the interview (will keep my fingers crossed for you).
Book a solicitor appointment as soon as you can and do a financial audit - make sure you know the ins and outs of your financial situation (bills, insurances, pension, savings, debt etc.
Birth certificates, marriage certificate and passports with your DM for safe keeping along with anything with the remotest sentimental value to you.
In your shoes, I would respond to his (quite frankly ridiculous and insulting "fight for me") message by email, block his number for the time being - you don't want him badgering you. Just say something along the lines of "At present I have nothing to say to you other than do not contact me directly, Susan will arrange for you to see the children". Speaking of children, I would also let School and Nursery know the current situation.
Delegate anything you can - Let the people who want to help you, help you. Get them to arrange four weeks worth of food shopping, book your solicitor appt, help you sort out your cms claim, sort out single person council tax etc. basically if there is any admin/life admin task you can give to someone, do it.
Just because he has asked you if you want to fight for him, it does not mean you owe him an answer. As others have said, he wants his ego stroking.
The decision as to what happens next is yours, not his. Unfortunately, many of us have been in similar situations, so as a result the experience, guidance and kindness on here is nothing short of amazing. Take none of his shit, my love, take your time, take care of yourself and if you want to move on from this stupid man, you can absolutely do it.