Just to correct you: The X-Men did not save Marvel comics in that era of junk bond financing, etc. The situation was far more complicated than that and was covered in a book some years back. I've read some of the details on that era. What really saved them is during that crisis is someone rescued Marvel from bankruptcy and that was Ike Perlmutter (who is still on the board today) and Avi Arad. Another thing they did was to sell some of the titles they thought would be the most valuable to the film industry. So they picked ones that they thought would be the most valuable like Fantastic Four, Spider-Man, X-Men, Daredevil, Hulk, etc. You should read the book, Marvel: The Untold Story for the details. It's very interesting. But to say the X-Men alone pulled them out of bankruptcy is false. They were going under for different reasons that go back to the time when they were owned by Ron Perelman and the whole junk bond era on Wall Street.
This article goes over it pretty well. It was an era of speculation and some fans were buying multiple copies of the same comic on the speculation that they could sell some on the secondary market and make lots of money. It's still going on today with variant covers. Look at the big number that some #1 titles get with variants. Now tell me how many of those continue with that same number in the following months when there are no variants. They are speculating on making money of the uniqueness of the covers and even that is fairly risky IMO.
As for those X-Men sales you mention, it's widely known that in that speculator era of comic books, a lot of readers were buying multiple copies of issues. When that bubble burst, it lend to the dire financial situation Marvel and the comics industry in general found themselves in. As noted in the article, by 1993 sales tumbled by 70 percent and it put some retailers out of business and Marvel as a company was endangered, as I described above.
As for moneymakers, I think it likely that you may want to remember a lot of the success that Todd McFarlane had with Spider-Man, which enabled him to leave Marvel. I think when you look at the macro version of things, going back to the 1960s, Spider-Man has been their gold mine. Syndicated newspaper column that has been going on for decades, successful cartoons, merchandising of toys, apparel, etc. Claremont has not found much success outside of his Marvel stuff. His X-Men run is one of the most popular but sales were juiced up by the speculator era of comic book sales of the 1990s. So there is no direct correlation between sales and actual number of readers.
And you are right that the relationships of Reed/Victor and Charles/Eric (or whatever his real name is) are different. But the point is that Claremont added a prior relationship much, much later that was never indicated before. We get a Reed/Victor connection right off the bat in FF #5.