That's because neither are required to actually run Mailman.
They are part of the 'mailman suite' that I installed, as an integrated whole.
I subscribed everyone via postorius, I'd have thought it would make sense and presumably be straightforward for postorius to create django accounts for them at that point.
I can also set the welcome message via postorius. Again I think it would make sense to be able to tag the postorius URL at that point as a default (perhaps using a text box rather than URL via third party website).
So I guess it's better to have the default not send anything, since the other way round would mean sending dead links.
Not if you're subscribing people via Postorius in the first place.
Currently the core component of Mailman doesn't have a way to know under which url Postorius is reachable...
Perhaps the web interface url could be passed in from the web interface?