Well, batspit.
I just ran through your checklist, refreshed my site (locally; I don't have a copy online yet) and now it works. I love it when that happens; I didn't change a thing.
So, a couple of questions:
1. since the Pages are in the DB, I only have to change their template once (not for the parent and child themes), right?
2. is there some way to batch change all my Pages to use the sidebar template? I have over 100 of them, and would prefer not to do them individually if possible.
3. is there a way to make the sidebar template the default for all new Pages I create?
Thanks...
EDIT:
In explicit answer to your question, I'm using the sidebar template; I haven't tried using either the plug-in or the code edit. I'm hoping to avoid the latter.