Just thinking out loud here,
I've had much the same thoughts, Robin - check out the way Elfrieda recruited Luella for the 'classes'. I read a few reports in the papers of people saying she was really good girl, but was a little obsessed with spiritualism. She was also selling those encyclopedias hard - even tried to sell some to the ticket lady at Highland Park the day she was hurt.
Elfrieda did book five months of classes, originally, but she had been seeing Hitch for the whole four years he'd been in Lake Bluff. In fact, they met up around the time he moved there, seems he didn't waste any time.
Motive -- pick one? Place was full of 'em, potentially.
1. Jealousy. Mrs. Hitch. (over Mr. Hitch/Elfrieda)
2. Jealousy. Mr. Hitch (over Elfrieda/Luella or some other person)
3. Jealousy. Luella (over Elfrieda/Hitch)
4. Revenge. Luella (over being dumped)
5. Convenience. Mr Hitch. (maybe he was tired of her demands)
6. Convenience. Barney. (maybe he was being blackmailed)
7. Fear. Mr. Hitch (maybe she threatened his marriage/to expose his crooked activities)
8. Lust. Chris Louis. (maybe he wanted what he couldn't have?)
-- I only include him because he, too, had a key. Not a serious contender at this point.
9. Loyalty. Frank/Oscar. (maybe one of 'em did it cause Hitch told him to)
etc, etc
Good contacts at the FB site. I'll have to make one to use as a contact, though - not using my personal one for crime sleuthing, even if they are all dead now. :B