It’s possible that I was missing a step after exempting imgur from downloading. Attempting to edit the post with the imgur hotlink was not working until I hit Rebuild HTML on the post with the exemption in place. After doing so, the hotlinking is working.
I don’t need the hard sell on Discourse’s upload/auto-download hotlinked images feature. I’ve looked at it, it’s great for some use cases. Mine is not one of them. I do want my members to be responsible for hosting their own images, there are many reputable image hosters on the internet and most of the members have years of experience with them. This method works and now that imgur hotlinks correctly, it’s what I’m most likely to pursue.
It’s worth noting that the issue of imgur’s aggressive rate-limiting has not been resolved or addressed at all. The auto-download capability by Discourse is rather crippled in this manner, and there’s been no suggestions here of how to fix or alleviate that. The lack of help there is disappointing.