... the trouble with ctl-w-ing things is the annoying desire to, probably unnecessarily, ctl-s them beforehand, too.

Ctl-S saves the entire epub. Ctl-W just closes a window (tab) of an individual html file in the epub. For the truly aggressive you could use alt-f4, and sigil will ask if you want to save before quiting.

And to confuse matters even more, all of they key shortcuts are user re-assignable using Sigil Preferences so not everyone will use that same keysequence shortcut in the same way.