Actually this is all in the Woodenwidget plans.
The single central roof beam isn't really strong enough to support the weight of the whole upper section as it's held in place with a single screm each side and a blob of sika.
What you can't see in this pic (but can in previous pics) is the two stringers that run the length of each long side and slightly protrude on the inner side.
The cross brace slots in underneath those stringers and so distributes the weight .
The diagonal strings are there to help keep the lifting mechanism vertical when at full extension. They just unhook when the lifting mechanism is removed.