To understand diamond upgrades you have to understand that upgrades occur at the external function level. You can add external functions, you can replace external functions and you can remove external functions. That's all. So to understand upgrades you need to think in terms of add/replacing/removing external functions.
Nice! I've been mentally modelling this as a bipartite graph between state variables and external functions with a 1 to 1..N relationship.
Seems like a lot of the insight/patterns of managing diamonds will come in handy with transient storage too.