All theories are canon.

So, I think I've solved the mystery of the AH insert based on all of the clues we've been given.

IDE: We have actually been completely mislead about the way that the AH author insert's timeline has gone. The whole DOTA scene with AH in the Sepulchritude shirt proposing to Vriska in the desert is actually the earliest point in his timeline that we've seen so far, and he has been in the dreambubbles the entire time--AH getting gunned down by Lord English (dressed like Calmasis, which I'll get into later) is the last point in his timeline (as far as we've seen, anyway). It doesn't actually make sense for the DOTA scene to have taken place after he got killed by Lord English, because we already know that ghosts generally appear in the clothes that they died in.

Think about it--the Hussie mansion, as far as we've seen, is in the middle of a vast desert full of horses, next to a big ocean. This is actually exactly where he is now, in the exact same clothes we were first introduced to him in! The "fourth walls" he posesses don't actually exist in some kind of outside-of-the-story space... they're just ordinary fenestrated plane portals like Roxy has, and just like with Roxy's portals, the space between them that Jade and John pass through is in the dream bubbles, with the trip taking 3 years because the distance between the sessions is so great. Any weird goofy things that happen during the AH segments (like the wolf head showing up in the attic, or Falcor barfing on bullies) can then be explained as just dreambubble magic.

IDE^2: Calmasis is a combined amalgamation of Alt!Calliope, Davepetasprite^2, and the other half of Gamzee, created as the opposite counterpart of Lord English, and Calmasis and LE are the two mirthful messiahs. This is why Calmasis appears to be wearing Calliope's clothes, Gamzee's grey skin, and Davepeta's hairstyle. Like, look at Calmasis's hair and Davepeta's side by side... they have the little curl and everything:



Later, Calmasis and Lord English (the two mirthful messiahs) combine to create the AH insert, which is why they look so similar but not identical. SOMETHING has to account for AH's love of horses, and that's bound to be the ARquiusprite part of him.

Alternate IDE^2: Calmasis IS the AH insert, and is created when they use the house juju against LE, which forces four individuals to combine into the four slots of the house juju: Lord English, Alt!Calliope, Davepetasprite^2, and 1/2 of Gamzee. They defeat him by transforming him into a more stable form, made of a perfectly balanced combination of brothers, moirails, two halves of a whole individual, and the Cal twins. Calmasis then ends up either painting his skin orange and donning a brown wig, or else changing his appearance via dreambubble magic.

So basically, the full timeline would go:

> Davepetapsprite^2 is inexplicably drawn to the battle against Lord English in a desert full of horses, because subconciously they know they're meant to be combined down the road
> At some point alt!Calliope and 1/2 of Gamzee show up as well
> Somehow in the final battle all of these components combine with Lord English to create the AH insert, possibly with Calmasis as an intermediary step but possibly not
> the AH insert remains in the desert full of horses, but because time makes no sense in the dreambubbles, he runs into Vriska much earlier in her timeline and try to give her the Ring of Life (which is just the transformed Ring of Void that we see Caliborn with prior to becoming Lord English!)
> the AH insert then does a bunch of dumb stuff in the dream bubbles (as seen in Openbound) and then follows Meenah/Vriska around, leading him to the ghost army battle against Lord English
> some crazy battle stuff happens, the AH insert possibly watches his own creation happen because of dreambubble time weirdness
> the AH insert decides to stay in the dreambubble desert and creates a mansion (or one is created from dreambubble memory magic)
> the AH insert faithfully documents the craaaaazy tale that is Homestuck, retaining the exact same narrative control powers that Caliborn already possessed, but using a combination of the art styles of Calliope, Caliborn (right angles = aliased pixel style), Davesprite, and Dirk, and using Homosuck as a sort of "rough draft"
> the AH insert eventually gets gunned down by Lord English, fulfilling the longstanding tradition of past selves hating their future selves and vice versa
> Either something yet to be determined happens with the AH insert after death, or he's just straight up double-dead after that and that's the last we see of him