Tangential, but regarding footnote 7 (efficiently recomputing cells, assuming no cycles), the correct algorithm for determining the order is topological sorting (https://en.wikipedia.org/wiki/Topological_sorting). Using a breadth-first search gives a good approximation in many cases, but there are pathological cases where it can produce an inefficient order.

> Uber Sheets

Hee hee, this name is funny on a number of levels :)