Index Allocation Maps
They are part of a mixed extent, the diagram just didn't draw a grey box behind them, probably more to keep it from being too busy as much as anything.
Data File : Page
Again, I think this is just meant to simplify and not make the diagram too busy, as well as not give any sort of suggestion that pages will necessarily be linked in a certain order. Surely you can picture the arrows that would represent a doubly-linked list? 


For most of this, you're much better off asking the author of the book and creator of the diagram, than a community of peers.