This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Potemayoids assume mobile form only after they run out of food
This explains why the spawn we see at the end of the series are juveniles instead of full-grown adults: they didn't have near the food available, so they switched from their fungal stage much sooner, before they had the time to build the biomass for complete bodies. This assumes that the fungal-animal phase transition doesn't put a end to all development(notice how Potemayo never shows any signs of developing Guchiko's horns) and thus the spawn don't simply have stunted growth.
Sunao's late mother somehow found a way to contact his son and friends from the afterlife (like Kanata Izumi) and dedicated herself to be the Posthumous Narrator. It can't be a coincidence that they share the same voice actress.