What's the difference between the latter two? Apart from that, it sounds like a gitops thing "deploy on push", while trying to meld the mutating pets with declarative cattle somehow.
Nickel vs. Nix? as I wrote, Nickel is a "statically-typed successor" - newer, created (in progress) by people with Nix experience, vs. Nix being dynamically typed.
As to the rest - more or less yes, nicely put :)
No I mean your last two list items, starting with "vs".
"Last recorded" is intended to reflect what was previously observed on the machine.