Wait there's a crossover between Evil Dead and Phantasm??? Is that official?
Yes.
Further Excursions was published by
The Sentinel: A Phantasm Phanzine and carries a copyright notice that the
Phantasm property is used with permission; Don Coscarelli is acknowledged for "his continued support, enthusiasm, and inspiration," and part of the content solicitation at the end says that materials submitted can be "for possible use" by "Don Coscarelli and Starway International, Inc." One of the contributors, Kristen Deem, was successively a storyboard artist, script consultant, and script supervisor/property master on the first three sequels.
Elkin's "Red Planet" can still be found online
here. As nearly as I can determine, Keene's "Hell at S-MART" is the only story to be reprinted (in Keene's 2003 collection
No Rest at All: The Definitive No Rest for the Wicked and in the fiction archive in his Patreon).
The Conjuring is not the same universe as Amityville because Amityville and The Conjuring are just using real events and real characters as inspiration. The upcoming Amityville movie might be, though.
As I said above, we can't say they're directly connected, but it is still worth noting that the two series rhyme even if they don't harmonize. I don't think it's too much to say that a version of
The Amityville Horror exists inside
The Conjuring Universe, albeit in a form we don't precisely know. There are at least three circles in this Venn diagram, but we don't know how much they overlap.