Rated 4 out of 5 stars

Great - but it seems that I have to choose this every time I view the message. Is there any way to set this persistent, and for a whole folder, not a single message?

This review is for a previous version of the add-on (1.1.1). 

No, the setting is forgotten when you switch between messages. The Thunderbird team decided to remove the "per folder" setting, see: https://bugzilla.mozilla.org/show_bug.cgi?id=1671880 (the folder charset was removed in part 2 there). For news, the server default charset is used. If you want to discuss the matter further, contact me via e-mail.