The Wikipedia page for Dr Xavier's school gives its address as 1407 Graymalkin Lane. But the Google maps satellite view of that address looks nothing like the school. And the movies use this English estate the set for the school. Why did Marvel choose this actual address? Did a Marvel employee live at this address?
Your supposition was correct. This address was chosen by X-Men writer Chris Claremont, an alumnus of Bard College, New York, a short hour's drive from Westchester. It's likely that he would have been able to physically visit the supposed location and pick out a nice quiet street to situate the (fictional) location of Xavier's School of Witchcraft and Wizardry.
Chalk it up to Chris Claremont. A comics writer who took over the then-failing X-Men franchise in the 1970s and brought it back to fruition by penning several classic story arcs and focusing on character development, Claremont attended Bard as an undergraduate. “What’s the point of going to a great school if you can’t make it a slightly supporting character in a cool comic series?” he told Almanac. Claremont admits that making Annandale Jean Grey’s childhood home was his call, but says that he wasn’t aware of Professor X’s relationship to Bard.
Claremont recognized the importance of tethering the fantasy world of comics to the real world. “The location of Xavier’s School in Westchester County is something originally established by Stan [Lee] in the opening issues of the series,” says the author. “My contribution to that rubric was to take his general placement and give it more specificity, creating a street address and a real sense of place. Its proximity to New York was a matter of convenience and publishing policy: Everyone at Marvel was located in or close to New York – unlike DC, where the JLA [Justice League of America] characters were placed in fictional analogues for the real world (Metropolis, Gotham, Star City and the like). This way, writers and pencilers could frame the story’s events within a common physical reality familiar to both creators and readers. Everyone’s writing and drawing and reading what they all know, which (for me, anyway) makes the adventures that much more plausible and a whole lot more fun.”
As to why he chose this specific address, it's worthy of note that Claremont cites Shakespeare as one of his key influencers. It's certainly possible that he was attracted to the location because of its connotations with Macbeth, Greymalkin being the familiar of one of the three witches, a spirit disguised as a humble cat in the same way that Xavier's academy belies a secret identity as the home of the X-Men.
First Witch: Where the place?
Second Witch: Upon the heath.
Third Witch: There to meet with Macbeth.
First Witch: I come, Greymalkin!