JenJam - How dissapointing. After all you have put into that place this is the way they treat you. Yes, you could kick-up a fuss/sue or take whatever legal action is available, but you might want to think about where this will leave you in the end?
Doubt this will make relations better and will certainly make them worse and in a small company that could be very, very difficult. In addition, do you really want the stress of taking it further?
Two things: Stop sending e-mails to your boss. E-mails only serve to escalate the problem. You need to talk, ideally face-to-face, but if not then at least the telephone, to get some clarity.
I know you have tried to be helpful in pointing him to the Mat. Alliance for more info, but have to say if a member of my team sent me an e-mail to this effect I might take umbridge as it assumes I don't have the basic intelligence to find out this info for myself. This sort of thing comes across so much better in a one-to-one conversation. E-mails can simply look and feel adversarial, regardless of the intent.
Secondly, regardless of what you agree now, i.e. going back after 6 months, you can always change your mind after baby is born. You only have to give your employer a set number of weeks notice of your intension.
Thirdly, sorry to say it, but it does not look like your boss/this company is really keen for you to go back. They are not making any effort to make things easy for you now, let alone when they have had six months to get used to you not being around.
Is this really the sort of people/company you want to work for? Maybe, after your maternity leave, you can move on to somewhere that values your contribution rather more.................