This is a great post, and an excellent example of the kind of analysis I want us to bring to the comic page. One of your most important insights, though, is made as an aside:

Not only is this sequence thematically odd for "Garfield," it is artistically experimental as well. By introducing an actual vanishing point perspective, Davis is putting Garfield in a virtual 3D plane that, like the introduction of "reality shaped" panels in "Vacation Time", frames a scenario in which the survival rules are not as cartoony as normal. In practical terms, this kind of move renegotiates the readers' relationship to the narrative diegesis of the story.

It's also interesting, in light of redheadedsnippet's post on color, that the version you provide here is colorized. Whenever I've seen this sequence before, it's been solid black and white, which I think gave it a more Kafkaesque feel. Have you seen it in that version as well?

The break at the end is, I think, the strangest part of it all. Not only does the narrator break the diegetic frame (which is kind of appropriate since this whole series is about framing), he deigns to lecture us about our imaginations. The strip is, at this point, already heavily metafictional, so the direct address from the narrator seems like overkill to me. I'm sure he felt that the work demanded some explanation, but I'm sure this answer still left some regular readers perplexed.

Also, I like your title because it suggests something that hadn't occurred to me before: what if this strip is actually canonical and the rest of the Garfield run is really Garfield's undeath? I mean, the repetitious and contrived gags have a certain sysiphusian monotony about them, and thinking of Garfield as an indifferent shade doesn't seem too far off.

Finally, I just want to mention that a Garfield Halloween cartoon scared the hell out of me when I was a kid; I guess I would have seen it a little before this strip series came out. I remember not really "getting" Garfield, so I didn't care much about him as a character. But there's this part where Garfield and Odie are hiding in a cupboard and the pirate ghosts suddenly throw open the door. As far as I'm concerned, that ranks right up there with Large Marge in the freaking-out-little-kids department.