Sorry for the delay, long weekend
1) I'll look to see where I read it. Or maybe I inferred it based on the mass targeting of mutants that happened prior to the branch to all super heroes. But I would say that Hellfire Club did not oversee to execute the massacre at Xavier's. No writer, Claremont included, brings this up again in any retelling of what happened. In every mention after this comic, and before, it has been the government executing a raid with intents to kills or imprison the mutants residing there. Even if they did use their clout, which is speculation, they wouldn't have wanted all the mutants there killed as they would seek to bring them in and control them as shown by having their own academy by that point.
2) It's possible that was the plan Claremont may have had, but we don't know sadly. Lots of his nuggets of future seeds either got ignore by future writers (or himself) or retconned. Given the memories shown by Rachel's rebirth in Excalibur 64 (mental influences removed)and the Phoenix Force's retelling of their meeting in Excalibur 52, we know she wasn't Phoenix before their meeting post-the DoFP story. So this encounter with the SK is false.
3) That's something I'm always weary about as one would expect that a multiversal entity would have better luck taking over the world if they were experiencing everything lol. The 616 version has skipped around different dimensions/universes and always comes back here. So I guess it would make sense from a perspective of it remembering Rachel and the Phoenix, but it's a thread that was never picked up on again, even with this reality's SK trying to subvert Rachel in New Excalibur he doesn't reference True Friends of meeting Rachel and Kitty or Betsy (weird, but another instance of Claremont not using his own previous stories).
4) As I think more about it, given that Ahab hadn't been created yet it would make sense he's not referenced (but will be shortly in the Days of Future Present) but it's something that has never been referenced outside of that one Excalibur issue. Ahab has always been noted as the one who broke Rachel's mind. Not the Shadow King. Additionally if the SK had done some level of programming in 811 then it would have been picked up on in True Friends and subsequently by the Phoenix Force when it rebuilt Rachel's mind from it's shattered remains to it's natural state. Never mentioned. It would be an interesting wrinkle but the credit is all Ahab's at this point.
5) I think that might be true as a means of Claremont editing his own tale for future storytelling. Moira still could have died, but Xavier didn't.
6) Oy what a headache. A different Bishop from a different timeline (which mimics his) where he goes to 811 instead of his own timeline's past? It's a lot of hoops for Claremont to jump through to just to use Lucas but sure. While I haven't read it yet, the story feels like it doesn't add value to the existing tale and also sounds like it removes Rachel's own agency of breaking herself out of the hound state and instead is sided by the "X-Men". I would love to be proved wrong.
When I looked up the issue summary from
UncannyXMen.net this tidbit from the editors makes sense to our conversation and to why it's not referenced:
"The backstory of Rachel and the Shadow King (including such elements as to why he knew her even before her parents’ birth) was supposed to be told in Excalibur Special editions #3 &4, which were, however, never published. The story was eventually published (though probably in somewhat altered form) in the limited series X-Men: True Friends, in which Rachel and Kitty inadvertently travel back in time to the 30ies and become embroiled in a plot by the Nazis and the Shadow King. What makes this somewhat problematic is that it happened in Earth’s 616 timeline and Rachel (and her world’s Farouk logically) is from a different timeline. Perhaps this, as well as several other inconsistencies – such as teenage Rachel already displaying Phoenix powers – can be explained away by the messed-up memories she has ever since escaping from Mojoworld (as explained In Excalibur: The Sword is Drawn)."