So if you have to work until June 30th, 2015 to get the $27,500, you really only have to go back to work for 4-6 weeks, depending on when you leave - and I know several mothers of twins who didn't have to go on bedrest or were able to work up until closer to their due dates than 30 weeks. That's well within your husband's ability to take 12 weeks for FMLA, so he could take over while you power through. I would say it's worth it, that's really not that long. Plus, that could be your "trial" time where you go back and see how you feel about it.
A couple of other points: you say your current bosses don't like kids, but that doesn't mean they will actually make your life harder for having them. You won't know until it happens, so don't borrow that trouble until you run into it. I also second the suggestions made earlier to look at other managers you might work for or seriously consider those recruiters if you want a better work-life balance. It sounds like you are very lucky in your work to be so employable and recruitable, I would leverage that as much as possible. You might find that it works best to find a new job in a lower-stress situation that pays slightly less but has more flexibility in time. And with two, I think the nanny definitely sounds like a good plan, at least at first.
You mentioned that you don't qualify for FMLA because your office is small but it's part of a larger company - I don't think it's the size of your office but the entire company that counts for what does and doesn't qualify for FMLA, but I don't know how you are structured.
I have a 4 year old and an almost 2 year-old, with number three just barely on the way (April, if everything sticks). I have worked the entire time other than maternity leave and I feel like I'm a better person and mom when I'm working - everyone's life is more structured, I get frustrated less, am more patient, and my kids have learned incredible things that I would never have thought to teach them as a stay at home mom. Plus, I am able to do great things at work that are very fulfilling - I recognize that's not the case with everyone, so it's a big factor in why I like my current arrangement.
I second those who say just wait and see, now is not the time to make any big decisions - you have had enough change thrown at you! Even though with twins it's coming up faster, you still have a LOT of time to think and decide. Save, save, save, and keep your options open. Don't let pregnancy hormonal emotions get to you!
Congratulations on the twins (my husband would flip if that happened to us in a few weeks when we get the first ultrasound, I can't even imagine!) and it will all work out!