Welcome to FT!
I see four seats available on the UD - 60B, 62A and 64B&J. Where did you see the map with only one available seat?
64A is actually taken, so it would seem that there is an infant on the UD! You might not want to be in 64B!
Many of the pre-allocated seats may well end up changing their plans...
There really is no way of knowing if you are going to be lucky until T-24.