Maybe require a Linkshell to register before being able to post for progression? You'd at least ensure they're an actual static group for 2.4, and not just a PF group or a 6-7/8 FC group who pugged two randoms for whatever reason. Require the registry at least 24 hours before any clears are claimed, so that people don't just 'shell up and register immediately beforehand just "in case" they beat it?
I like that idea. I'm not against having Linkshells having subs available in them either but at most i would say 4 players. (Not specifically this make up but my reasoning behind the number 4 is 2 dps/1 healer/1 tank sub or something of the sort.)

Seeing as we never specified rules and regulations prior to your post, I'm not against listing your LS kill, however those changes would have to be made for future progression.