From what I know (which isn't much detail) that document is indeed required to be signed by the three sets of people. Two witnesses are required. I believe civil partnership has the same requirements.

I don't see that it's completely out of the question that someone could be tricked into signing it but I think there's some official wording required for marriages too. If it was part of a performance? Might work. Although if they had proof of this afterwards I imagine it's grounds for divorce.