Phee - I really don't know anything about this, so I hope my post is not massively unhelpful but I suppose I just wanted to air some of the things I would want to say and then I'm sure a more experienced/knowledgeable poster can say if/why it's not appropriate.
By the time of mediation it will only have been 6 weeks. You absolutely should not feel that you have to agree to something you are really uncomfortable with. It needs to be stressed in front of the mediator that it has only been 6 weeks since you were blindsided.
It is far too soon for to be discussing 50/50 access; for toddlers to be spending EOW away from their mother who has been a SAHP and primary carer their whole lives is already the very limit of what you think is appropriate for such small children who are already having to adjust to the huge change that he has imposed on the family. Yes, not being able to spend time with your children whenever you want to is a huge hardship for you when you have done nothing but be a loyal wife and loving mother; however, you are thinking about the welfare of your children first and foremost and you want to know that he is fully settled and established (say 6 months or whatever you decide) before you would feel happy about inflicting further irreversible change on your children (i.e. new living arrangements and new people in their lives).
I would just calmly keep pointing out that, for you and the kids, it's only been 6 weeks since your lives were turned upside down without warning. You need to find equilibrium before the family can cope with more changes.
"H, please be reasonable (which he will want to appear to be in front of the mediator) - 6 weeks ago the twins had a stable family and home and now that's gone. They are already having to adjust to a huge change - I'm asking you think of their well-being and to wait 6 months before disrupting their world even further. If this is a permanent change for life, then we should take our time - we spent 9 months preparing for their introduction to their parents and home, it's only right that we should prepare properly for this change after the shock we experienced as a family just 6 weeks ago."
As Rosie said, I wouldn't mention work to him until it's all signed and sealed and you've also had a few weeks in the job to make sure it's a permanent change for you. But, also, as she said, you could point out that he would need childcare at some point, which is yet another change for them, and so you might start introducing them very slowly and gradually to some childcare as they are now eligible for the 15 hours.