With the slope of the yard, I can put more rink over the bottom part of my deck (about 2 extra feet) without much more total water...the bottom segment is the same height as the highest part of the yard.

Question: is there any reason not to do this? Will it heave strangely or behave differently from the rest of the rink?

This is Year #3, and I'm looking to make as big a slab as I can in our urban backyard...

Are we talking about a wooden deck or an interlock stone patio? I would be less hesitant to cover a stone patio with the rink, depending on the substructure and how well it is packed, than I would a wooden deck.

Wooden. I elected not to do it since it has some areas that come to a pretty sharp point.

Unless you covered it with plywood or something to protect the tarp (and the deck), I think you made the right decision.