Excuse me if I am misunderstanding something.
I don't pretend to know all the inner workings of TS, but I think this is backasswards. You need to delete all snapshots backwards in time consecutively, back to a certain point. You can't selectively delete snapshots earlier in the sequence as they are all linked consecutively.
The first snapshot base is linked to the next snap, etc, etc. You can only delete from the end in a consecutive manner backwards to a specific point. Then you can take new snapshots which will then be linked to the last snapshot which was not deleted.
My understanding is you cannot just delete the ondemand snapshots if there are intervening scheduled snapshots. All snapshots must be treated the same in the base timeshift folder and be deleted consecutively backwards by timestamp regardless of wether they were ondemand or scheduled.