Actually, the directions on https://www.torproject.org/docs/debian.html.en work okay. I was trying to automate things with Ansible, but the format changed at some point, from something like:
apt_key: id=A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89 url=http://keys.gnupg.net/pks/lookup?op=get&search=A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89
to:
apt_key: id=A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89 keyserver=keys.gnupg.net
The URL at /pks/lookup no longer exists, so I saw a 404. Using the newer format with just the hostname of the keyserver it works okay.
Regarding http://keys.gnupg.net I still don't know why there is a SSL mismatch in the browser, or why you can no longer access the web UI, but it's not as broken as it looked.