Sorry forgot this one, mail notification settings says
"Send an email if a new chat message arrives and I am not online"
And I receive one even if I'm connected (but not on same tab) which does not match option configuration 😉
But perhaps I should add a not in the sign up agreement that "by signing up you agree to receive emails from us" or something like that, to be safe.
If you're in the EU, this wouldn't constitute as opt-in. The EU states that a user must specifically click a button that states "I want to receive these emails". Unless it's regarding their account. So, if the daily digest included notifications, it would be fine, but if it was just new topics, I'm not so sure. It should be fine either way, as you've said.
The second one could probably be achieved with 3rd party tools already. The best one I can think of would be IFTTT. Each category has an RSS feed, therefore if RSS feed gets updated. Send email. Full IFTTT support would be great, I'm not sure how that would work, but being able to set up that sort of thing would be bloody brilliant. Maybe even baked into nodebb, so we can set up "tasks" similar to how stamplay.com works. Just with a betteR UI.