Ok, i know about the rituals thing, i actually did that intentionally; its a two step process where first a ritual is created/performed, and then after that dies a creature can be summoned from it. That isnt final by any means, but it does serve a purpose; since the rituals are immobile, harmless and will quickly suffocate they arent a huge threat to the player, which is important because the creatures summoners actually "summon" have magical powers and attacks of their own. Imagine walking into a fire summoners tower and being pelted by hundreds of fireballs simultaneously. Not a chance in hell youd survive even if you were a legendary dodger and shield user wearing full coverage armor and nothing flammable; something would get through just because the rng hates you today. So once they start suffocating the summoners can create a smaller number of summoned creatures that pose a challenge (i at least get killed by them all the damn time) but arent overwhelming. They also come in unique castes which prevents summoners of one type (fire) from stealing corpses already in the ritual stage that would have been used by their respective summoner (wind, say)
Ive never heard of one secret-bearer making multiple groups before so maybe thats a thing in the new version? I certainly didnt notice that but ill have to check legends mode on my saves again. I know a deity can create new slabs for new worshippers whenever, but it sounds like you mean the same mortal people having multiple apprentices. And yes, some of them never get the chance to build a tower, because they start out in a camp and have to collect so many corpses from worldgen battles and graveyards (i think?) before the game gives them a tower and in particularly peaceful and small civs that might never happen. And not all of my secrets have raising interactions that even allow that, so theyll probably sit in their camp or stuff their slab in a library at a town or something.
Some of them not gaining the secret is probably a bug, i have no idea how that could happen but ill try to find out.
Update imminent, at least. Polymorphy is working well enough