commodity value persistence

I'm not sure how these statistics get saved into the gallery, but loading them is determined by the 'gallery_load_behavior' variable.
My assumption, although this is testable if needed, is that all statistics are saved no matter if modded or not, and only the ones that exist in the game can be loaded. That is because of different expansion packs between users, all statistics have to be saved in case they are from a pack, but only the ones that exist in the end-user game are loaded.

By default, all statistics are set to LOAD_ONLY_FOR_SIM, which means that all statistics are saved into the gallery (or between saves). This does not apply to statistics on objects, as objects do have statistics too, but only statistics that are set to LOAD_ONLY_FOR_OBJECT or LOAD_FOR_ALL will carry over.

There are only 26 statistics set to DONT_LOAD, but they are mostly space-task-specific, statistics that change depending on where the Sim is or what the Sim is doing. Interestingly enough, PlantSims Water need/motive is not saved, I guess their form doesn't persist through the gallery?

So to answer your question, the majority of the commodities/statistics keep their progress, even the modded ones.
Obviously I'm not a developer, so I might be missing some element of Gallery that purges or sanitizes data, which most likely happens to some degree, but this is information from where I can see it.

Then there has to be way more to this than I am aware of. The example statistic, with FIXUP_USING_TIME_ELAPSED, and decay disabled, should indeed not be reset back to 0. I wouldn't mind testing this and learning how it works, but I'm way too time-constrained. Maybe someone else knows better what's going on.