This may not be very helpful, but everything you are describing is why I left IT contracting to go permie in the public sector when I was pregnant. The stupidly long hours and weekend working are 9 times out of 10 a panic reaction by weak and incompetent management, who can't think of any way out of any problem except by flogging their workers harder. It gets to be institutionalised and it becomes very difficult for anyone to swim against the tide, for whatever reason.
A friend in one of my old companies has been sent off to Germany at virtually no notice for six weeks, missing Valentine's Day with his gf and an interview which was very important to her. I was staggered he even consented to go, particularly when the trip was put back a week "because there were no computers for them to work on". So, this is 'life and death' (in telecoms, not actual life and death) yet can easily be postponed due to absence of PC? He hasn't even put up much of a fight about it, that constant pressure to 'take one for the team' is insidious.
If your organisational skills were worthy of promotion, I'm pretty sure you can manage your team yet only work 9-5. What they don't like is the precedent it may set, I'm sure. Plus there will no doubt be tensions if the rest of the team are made to work more hours and you don't.
You've got one of the owners on side, you could play a political game, but I know I wouldn't have the energy for that. In all honesty, I'd be tempted to try just leaving at 5 without a word of explanation, and if it's likely anyone will need to speak to you, say you'll be contactable by mobile.
In terms of maternity pay, the sooner you leave the better position you'll be in elsewhere for next time around.