Here’s an unpublished story regarding the clubhouse origin. (No, NOT Fortress Lad.)
Did JLA 3000 tend up tying into LSH? Keith Giffen’s terribly annoying interview for Comic Vine said it would not: https://comicvine.gamespot.com/articles/interview-j-m-dematteis-and-keith-giffen-talk-just/1100-146800/
Sort of. An evil ersatz version of the Legion showed up as antagonists in JL3001.
I own basically none of their Adventure Comics appearances in their original format. But I’ve covered pretty much all of them with the B&W reprint Showcase series. (I actually do have a copy of the Super-Stalag of Space issue, but that’s it.) Just too expensive to find them in readable condition. Apart from Adventure comics, I have every core Legion series and mini-series completed, except I only own LSH v4 #125 in reprint right now. I’m still working on their appearances in other people’s titles - but I do have most of their appearances in Superboy, save for two issues. Once it becomes ‘Superboy and the LSH’ I have them all.
I have about two dozen singles to buy, and I’ll have every canonical Legion appearance ever.
I will also add, I am working from an extremely broad list of what constitutes a Legion appearance - three of the issues I am looking to collect are the Adventure Comics issues featuring Dev-Em, who isn’t a Legionnaire, but is a Legion supporting character.