"Primal Self" is the reason Garfield hates mondays.
Monday was when the events of Primal Self occurred. Whatever Garfield did is now trapped in his subconscious, and so whenever monday comes in his later lives, he's always dreading it. It brings up too many painful memories.
The book version of Garfield: His 9 Lives is the successor to the animated special version.
At the end of the cartoon, Garfield is granted nine more lives. The book is what those next lives contain.
"Space Cat" is not the first time Garfield has been given nine extra lives. Also, all of Garfield's lives from all three versions (including the BOOM! comic) are canon
I like to think all the lives are canon, the only trouble is Garfield is supposed to only have nine lives, and to included the lives not in the original book would mean he would have to have sixteen lives (the nine from the book plus the four added in the special and the three added in the BOOM! version).
It's possible that God likes Garfield so much, that he gives him an extra 9 lives every time he runs out. This could explain why lives 3-8 in the book are all from the 20th century even thought the first two are more vastly spaced out. It's because he's had many lives per century.
It's likly present day Garfield is the 8th life of that current cycle and doesn't know he's been given all these extra lives. As a result, he can only remember seven of his past lives and see one of his future lives in any canon.
"The Garden" is a representation of the thought process of a child with a strong imagination dealing with the death of her beloved uncle.
All the devices and decorations are based on toys and other gifts he's given her her entire life, which make up a large portion of her mental landscape/Happy Place. He went to the circus in her imagination (perhaps her mother said he went to a better place) and left all the amazing things he brought her behind. The box represents her grief and all the negative emotions she feels in regards to his death. Tempted as she is to fell into despair (opening the proverbial box and potentially ruining her Happy Place), she decides instead to focus on the joy knowing him brought her (he might have even told her not grieve for him) and chooses to be happy. As for the orange kitten, perhaps he's a real-world gift from her uncle, and the version of him in her imagination has Amplified Animal Aptitude.
Alternatively: "The Garden" takes place during the "Great Binge" of the late 19th-20th centuries, and Chloe is a mental patient
Chloe is a patient in a private, upscale late 19th/early 20th century mental institution during the 'Great Binge' and rather than travelling with a circus, Chloe's uncle was the one who institutionalised her. The colourful and whimsical surroundings are just a skewed version of the institution's extensive gardens filtered through the various psychoactive substances they put into Chloe in an attempt to cure her. Garfield's incarnation in this life is blissfully unaware of all this, just goes along with Chloe's delusions and hallucinations, and enjoys that he has an owner who can spend all day playing and cuddling with him.
"Primal Self" may not have ended how we assume
Nothing after that infamous panel is discussed. We assume that Tigger kills or at the very least harms his owner. That's most likely what happened, but other plausible things that happened were that the owner may have killed Tigger in self defense, or alternately, assuming the owner survives, she has Tigger put to sleep (which would explain Garfield's dislike of going to the vet). There's also the possibility that Tigger's actions didn't directly kill her but knocked her over and the fall being what killed her. Also, less likely scenarios are that Tigger attacked the electrical outlet instead or attacked something behind his owner that killed both of them.
That wasn't his old hidden animal instincts coming out in "Primal Self", that was an evil spirit possessing him
For starters, cats in the wild don't try to ambush attack animals much larger than them, or anything that can fight back and inflict damage on the cat. It would make sense for a big cat, or even a bobcat, to predatorily attack a human, but not a housecat. His true "primal self" would be more defensive, hiding, and hissing defensively while attacking and retreating. Feral cats in reality will when encountering a human stand their ground, hiss, growl, and lunge, putting on a show to try to intimidate the human away from approaching them (unless the human is interacting with a feral cat's kittens, then the cat will be far more forceful in driving the human away, but not through an ambush attack). Compare how a cobra reacts around a mongoose to how it reacts around a rat. Secondly, look at the wall. The wall cracks open, and a nightmare spirit shaped like a cat emerges, it then possesses Tigger whose eyes glow strangely. I'd believe it were metaphorical if not for him acting VERY unlike a feral cat. His fur also changes color. That is most definitely not Tigger inside of there. Most likely, Garfield/Tigger likely remembers it the way one would remember a bad dream. Reports of humans allegedly possessed by demons bear far more similarities to how Tigger acted than any feral cat.
