Was CASE FILE ADVENT sent from the surface of Planet 4?

Essentially the Mural was a Easter Egg

No. Easter eggs are in the background and do not play a role in the plot. In Prometheus, Charlie directly studies the mural.

I understand - it’s convenient to consider the mural to be an Easter egg, because it destroys David as the creator. The link to LV-426 from Prometheus additional materials may be canceled because it is not in the film, it is not on the screen. But the mural in the story, it's a part of the film.

I cant 100% Speak for ADF and the ALIEN Novel as its been like 30+ Years since i read it.  To my knowledge some of the Drafts did have the Dead Pilot (Space Jockey) but i am not 100% sure we know what Draft that ADF had based his Novel Off and Often we NEVER see the Drafts that those who Write the Novels look at.

I will have to look at the Alien Novel again, but as FAR as i am aware when working on ALIEN there was the idea of having a Space Jockey the Only Changes coming from making the Cargo/Egg Hold become something that was UNDER the Pilot Chamber that was NOT in the Drafts, and so as FAR as i am aware the Drafts had the Space Jockey.

I will say that those who work on the Official Novelizations would see a number of Scripts/Drafts but would be given a Specific One to work off.... and the Combining of the Egg Cargo and Derelict was kind of a Last Minute Change, and so its likely that ADF had already completed so much of the Novel by then, and maybe when he heard about a Scene being Scrapped he maybe thought it was the Space Jockey one and Omitted it, when in reality it was the Pyramid Scene that was Dropped.

Again i cant be 100% certain on that, i guess we have to see if ADF had ever said anything about there being NO such Space Jockey Scene in the Draft he worked of.

I do get the Impression that ADF does ADD stuff to his Novels that are NOT in the Movies or Drafts.... but then his Books Get the Green Light without him having to OMIT any additional content he adds.

Regarding Alien Covenant i think it depends on what Draft he had to base his Work Off.... who knows how many Drafts there was and WHICH of those he used..... the TWO that were Released, we NEVER had the Found as Was Egg Scene that ADF had in his Novel and so this is LIKELY something he added to the Novel that was maybe not Supported by the Drafts but again it was given the Green Light (his Novel) and so someone at FOX would surely had read it before allowing it to go to Print.

When it comes down to the Bombardment Scene this was in ONE of the TWO released Drafts, but Certainly it was NOT Scheduled to be in the Movie until Very Late and so its likely that ADF was made aware that the Movie would have NO SUCH Scene by the Time he had Nearly or Fully Finished his Novel.

Regarding the MURAL in Prometheus it seems it maybe never had any Purpose, it was NEVER in any of the Drafts.

There has been some Conflicting Reports on it... One Suggesting there was NEVER any intended Connection, and it was purely a TRIBUTE to HR Giger.  The other which was made by someone who worked on the MURAL had suggested again it had no real Purpose but that it was showing the Deacon.

So it seems there was NEVER any real reason or purpose to those Face Huggers, there was NO reason they was there to show any kind of Connection or to play a CLUE to what was going on.   By that those who Ordered the Mural to be Designed and those who Worked on it, NEVER had any Explanation or Story for WHY those Face Huggers would be there, they were Simply a NOD to HR Giger and so a Easter Egg of Sorts.

But we get things added to the Movie that done have a real baring to the PLOT, some of Davids Drawings are a Example.  Sometimes things are thrown in because they look COOL or they are Concept Works/Props that they just add in for the sake of using them with NO real Connection to the Plot or mention in any drafts.

You're correct Leto.  

My point in regards to the Space Jockey's absence in the book is that many consider the book canon with this major missing item.  If memory serves, Alan Dean Foster wrote the novelization for Alien in a few weeks...applause for that by itself.

In regards to how "canon" is labeled.  Alex White cites "canon" books that Titan is putting out (here).  

While I might be more quick to label a written piece "canon" Mr. White goes on to cover the official classification from the publisher in the article, below.

"So when it comes to presenting this idea to Titan Books – who has a lot of these extended universe “Alien” books going back to the nineties – how much do you need to line up with what’s come before? Is there a story bible, do you need to connect to the cannon films?

Yes, I had to pitch to Titan and the idea needed to fit into the canon books they’re currently putting out. So I couldn’t just pitch any old thing, I had to really workshop the idea with the editor, Steve Saffel, before sending the pitch to FOX who have to give final approval."