Hiya Forrest,
I would think if you told them to change it code wise, showed them the sections of the code to change, put really explanitory comments next to each area they need to change so they know exactly where they need to go, that they should be able to handle that. Especially if there's more than 1 of them that they can discuss it between. I would say at least one of them will get a good handle on it and be able to help the others understand and see it better so it should be good. I say the HTML Editor would be best because in the end it is really the easiest way to go for something like this.
Sounds to me that the main things will be, upload the new files, change the link locations. Put a nice big comment around the text box that they would edit to describe any of their new content and tell them to keep it simple and 1 paragraph or 2 (showing them of course to put them between the paragraph or div tags etc.). That should be it if that's all they are doing is updating Calendars and pdf or ppt files.
I think this would be a lot easier than trying to teach them something like VSD where they will need to use insertion tools for some things and they may not know what tool for what job type thing.
Just my 10 cents, but I would think it's doable