I know that it's difficult with you leaving in two weeks and being pg, but I would absolutely be following this up. Too often employees are bullied into doing what is easiest for the employer, when obviously the explanation they provided you with makes not one bit of sense at all. Not one.

If were you, I would be spending all my time on the phone, with ministers, workers rights advocates and your union (again and again and again until someone listens) to find someone who will go to bat for you. For your manager to refuse to sign your form is equivelant to blackmail, and sounds incredibly unethical to me.

I have a long story with regards to maternity leave rights which I won't bore you with, but suffice to say, I was told early on that I wouldn't win my fight with my (govt) employer, but after making a lot of noise, talking to many people, writing a lot of scathing emails, and being determined, I got my way in the end. I knew I was right in principle, and it took me standing my ground for others to take another look at how they were doing things.