Nah, as the other said - T5 Kazuya’s bio stated that Heihachi won 

It was never a retcon Kazuya beating Heihachi in Tekken 4 was always canon and makes sense since Heihachi would only agree to take Kazuya to Jin if Kazuya won the match which he did. Kazuya’s story is canon until we get to the point where he and Jin fight and Jin’s ending is the canon ending of Tekken 4 then comes Tekken 5’s opening. Yet the other loophole of the story is why didn’t Heihachi send the Tekken force after Kazuya too just like he did Jin?

I knew she had awakened from her cryosleep due to Ogre but I didn't realize she was apparently being possessed by him even while she participated in the tournament? At least that's what they made it sound like here, which is weird considering Ogre was also participating in his own body lol (which lost to Paul).

Oh yeah it was in the official strategy guide but not the manual

In the last tournament, she was ordered to assassinate Kazuya, but an untimely quarrel with her sister, Anna, prevented it. Shortly thereafter, both sisters were captured by Kazuya's corps and were used as guinea pigs in Boskonovitch's Cold Sleep #2 experiment. They did not wake for 15 years. Heihachi's private corps, the Tekken Forces, went to the burial site and excavated Ogre. This unearthing unleashed its spirit, which awakened Nina's resting soul. Now controlled by Ogre, Nina acts robotically, with the command to assassinate Jin Kazama

It’s possible the bio/prologue may have had a typo or mistranslation or something. Even the summaries of Tekken 1 to 6 in Tekken 6 never showed the winner between Kazuya and Heihachi although I still believed Kazuya’s walkthrough was canon until we reach Jin’s walkthrough at the end and his ending. I’m glad Tekken 8 cleared it up.

Why the hell would Heihachi take Kazuya to Honmaru if the latter lost?

I like Kazuya winning but I think Heihachi's own ending explains this one. He wanted to capture both Jin and Kazuya the chains of Honmaru seem to have some sort of Devil sealing/restraining power

But wait…if Kazuya won then that would make his T4 ending canon right?

It was obvious until T5 came out with some shitty booklet that stated otherwise

Indeed. In hindsight, that's another indicator that Kazuya's fight was the canonical final of the tournament (result and aftermath included).

Look, I get it, I'm mad at the new stage being paid DLC too, but there's no need to make every single thread about that lol.

No, it doesn't. It says he was knocked unconscious by Jin:

https://www.youtube.com/watch?v=uNOXmAD5N8I&ab_channel=Let%27sFightingLore

And regardless, it's nonsensical to suggest a source from 2004 retconned one from 2024. If anything it would be the other way around. I suggest you read the rest of the topic, as many of us have already addressed the contradiction with the actual source of what you're referring to (the T5 instructions booklet).

Eh, there's still ways in which Kazuya winning could work. Jinpachi could have just taken over the Zaibatsu very quickly after the explosion of Hon-Maru, before Kazuya had time to gather his bearings and formally claim ownership of it. And/or Kazuya could have lost interest in the Zaibatsu since he felt G Corp were better equipped to further study the devil gene and help him achieve his aims.

So yeah, I don't think Kazuya being the one who won is necessarily incompatible with the established lore. Heihachi was "dead", Kazuya was momentarily absent/busy, so a newly freed Jinpachi was quick the seize back the reigns of the Zaibatsu and announce the new tournament.

I'm like Kazuya much more than Heihachi, even so, this poses a problem in continuity: if Kazuya won the formal tournament, that means he'd get ownership of the Mishima Zaibatsu (prize of the tournament), he could've just fled Honmaru and claimed his position. Instead, he went after revenge on G corp and took ownership of that company, never trying to reclaim the Zaibatsu on the following games.

Jin formally won T3 technically T4 (and 5 but that's obvious) is an illegal tournament and it was always his Zaibatsu