So their expected value of "yea" is -550, while their expected value of "nay" is -700.

This is only true if the experimenter doesn't know the result of a coin flip (otherwise it's either 1000/700 or 100/700, but you don't know which). But how do you decide to model your opponent as being someone who doesn't know the result, rather than someone who does? The only way I can think of is to follow UDT and always specify that your opponent is in a state of complete ignorance. But once we've borrowed this rule from UDT it seems like we're just plain using all of UDT. We've just made it more complicated by sticking a minus sign on the utilities and then picking the least favoured one. The use of an "opponent" doesn't seem to add any insight.

Suppose I rephrase UDT this way: Visualise a version of yourself before you had any evidence. Do what they would want you to do. As far as I can tell, this is just the above post with the minus signs taken out.