I just stumbled over the seemingly popular interpretation that Mufasa's ghost was only in Simba's mind and that's why he didn't tell the truth about how he died. I was like 'Whaaaat?!' That never crossed my mind, in all these 15 years.
I guess it is kinda left up to the viewers interpretation in the film but Mufasa truly appearing to guide his son, fits the theme of the story much better than just Simba's imagination.
Rafiki had summoned him. You know Rafiki was able to do weird stuff, such as seeing that Simba is alive and where to find him, by doing something to the dust in the wind that had been sent out by Simba. And I think Rafiki's line about the strange wather, right after Mufasa's spirit was gone, implies that Rafiki saw him too - and so it indeed was not in Simba's head, but summoned by Rafiki.
It was perfectly calm before Rafiki tells Simba to look harder at his reflection. Then the reflection changes, coulds gather and wind blows and all that and suddenly it's all calm again right after Mufasa disappears. A weather being strange suddenly and exactly the duration of someone's "innet vision" would be highly unlikely as it would be too much of an coincidence + Rafiki wouldn't make a remark of the strange weather in such an all-knowing tone if he didn't see exactly the same Simba saw.
Also, don't forget that this is a Disney classic cartoon - not a nature document. Disney has always, always brought back the dead in some practical level for real. Exceot in Bambi but that's really unlike any other Disney classic cartoon in every which way anyway. If all else fails in order to resurrect the dead, they've used magic. In this case, the practice of summoning spirits.
And yeah, it's official that it was really Mufasa's ghost and not in Simba's head:
1.) On the commentary track the producer and the diresctors refer to Rafiki as a "crazy shaman character". Shamans could summon spirits, as said earlier. On the commentary track they also keep talking about "Mufasa's ghost" and not of 'Simba's inner vision'.
2.) In the "Story Origins" bonus featurette, the film makers talk about the Hamlet references and say "We had Simba's to-be-or-not-to-be moment when Mufasa's ghost visits him and then leaves."
3.) In the "Themes of the Story" bonus featurette, they also talk about how "Mufasa arrives in Simba's hour of need." and how Simba gets to do what we don't but would want to; 'touch' our long dead loved ones and receive guidence and strength from them.
Thus, it totally was Muffy's spirit summoned by Rafiki, coming to Simba, taking a physical form in the water of the river and then in the clouds. (:
So, since it was really Mufasa talking to Simba... Why didn't he tell the boy that Scar killed him?
Because Mufasa was a wise father. The truth would've filled Simba with hatred and rage and that would be blinding and distructive and not much strength-giving. As in, a wise father does not prepare his son to a great battle, by filling his heart with hatered but fills him with hope and courage, and with faith in himself and in his place in the world. That's what Mufasa did and that's why he didn't tell the truth.
Not to mention, that the truth didn't matter anyway, not in the big picture. As for Simba personally, he already knew it was an accident.
All they needed was for Simba to let go of his pain enough, which he was able to do by knowing his father still believed in him and still loved him, no matter what. And so, to go back to save those still alive of his loved ones, by claiming back what is rightfully his. He needed to make the choice out of love - not out of hate.
Love, hope, courage and faith in himself, all of that was in his decision to go and try claim the throne back, courage was also in him saying he has left the past behind and admiting to his pride that he was "guilty" of his father's death, faith in himself shows also in him declaring that he's NOT a murderer. His mother's reaction (which Mufasa could not have known) to his confession probably didn't help him keep all the strength he had received from his father, when Scar fired it up with his words of "everyone knowing why" Daddy isn't there to save him.
Simba stood much better ground and chance of success, when making the decisions and efforts out of love, faith, courage and hope, than if he had been driven by hate and bitterness. As said, hate is blinding, distructive, draining emotion. No good for anything.
Mufasa also wanted his son to stop running from himself and instead to take his responsibilities, and go for it by a path that pushes the boy to becoming a wise king trhough scary and difficult choices made with honor and love, and by putting his pride before his own feelings, regardless of what those feelings might be. Telling Simba the truth about Scar's part in his death (hence, filling him with hate and bitterness from the start) would've completely destroyed any chance of that.
In short: Yes, Mufasa's spirit was really there and not in Simba's mind. And Mufasa didn't tell the truth because he was a wise father, who knew what his son needs to carry on what he must do, and because the truth didn't matter at that point, not generally and not to Simba since he found out his Daddy still loves him and wants him to follow in his paw prints.
I've also posted this on the MyLionKing forum, under my username SuperBabySimba. Some of this origins from my posts at IMDB.com's TLK board, under my username VampireOutlaw.