How can I 'unwatch' a complete year's worth of a series I want to
re-watch without doing it one episode at a time?
Whether you’re trying to remove your history or just re-watch, this can be done on a show, season or episode basis.
So if you re-click the checkmark icon on a season, a pop-up will confirm that you want to remove all plays. Same for a show or episode.
For re-watching, go to your progress page, and click the rewind icon and a pop-up will confirm when you started re-watching and will start showing on your progress page again. (See Reset Progress on https://blog.trakt.tv/advanced-settings-8c2f31faf4e)