1. Let's see if I understood

badlop — Mon, 04 Dec 2017 12:30:11 +0000

1. Let's see if I understood the question...
Well, the Message element follows https://xmpp.org/rfcs/rfc3921.html#stanzas which mentions jabber:client,

Then the Fin element uses the NS defined by https://xmpp.org/extensions/xep-0313.html

And the Set namespace follows the definition of https://xmpp.org/extensions/xep-0059.html

Why each namespace looks different to the others? Due to historial reasons, I guess.

2. I doubt it would be configurable

ejabberd - Comments for "Namespace prefix configuration"