I completely agree with you -- I only consider Lovecraft's own works to be parts of this universe. Lovecraft and other writers actually started this 'game' in which they all inter-linked their stories, but I have to say those not from him I consider only some kind of glorified fan fiction as you say. I partially support Michel Houellebecq's 'circle' idea of disposing the Cthulhu Cycle, but I have organized my own version -- I divide the Cthulhu Cycle into three circles.
The First Circle is composed of the core stories of the Cthulhu Cycle, which Lovecraft wrote all by himself (meaning they weren't rewritings or corrections for his clients):
Dagon
The Nameless City
The Hound
The Festival
The Call of Cthulhu
The Case of Charles Dexter Ward
The Colour out of Space
The Dunwich Horror
The Whisperer in Darkness
At the Mountains of Madness
The Dreams in the Witch House
The Thing on the Doorstep
The Shadow over Innsmouth
The Shadow out of Time
The Haunter of the Dark
In the second circle, what Lovecraft wrote based on ideas or annotations by his clients, or rewritings of their stories:
The Curse of Yig
The Mound
Medusa's coil
The Man of Stone
The Horror in the Museum
Out of the Aeons
The Diary of Alonzo Typer
In the third Circle, Lovecraft's poetry and very short stories. Won't list them all, but I reckon the most relevant ones are:
Fungi from Yuggoth
The Doom that came to Sarnath
Nyarlathotep
The Crawling Chaos
Creature of the Loch
That's the general 'Cthulhu Cycle Canon' in my view. I would like to erase The Dreams in the Witch House and The Mound from it, since they are the two stories I don't really like (Medusa's coil comes close, though only for the odd end twist), but that would be rather arbitrary.