Addendum to original post: After waiting 24 hours (too busy to take on troubleshooting yesterday), I started by removing the battery from the Eve Weather and resetting it. When that did not work, I used the Eve app to remove the Eve weather from my HomeKit Home and then reinstalled it. The first time I tried this it did not solve the problem.
However, I went through the same sequence (reset, remove, reinstall), the Eve Weather showed up in my Home and functioned properly in the Eve app (was able to set it to F instead of C, unlike the first time). But the best thing is that doing the above ALSO made all my Nanoleaf lights reachable. It's almost as if having that one dysfunctional/unreachable device on the Thread network rendered the entire network unreachable. Weird!