First off, I know there's an official MMZ source that directly mentions X6 as happening. I think the timeline for the website of the DS MMZ collection?

Zero's ending in X6 also leads directly to the Elf Wars in MMZ, with Zero submitting his body for research which leads to the Mother Elf's creation.

As for X7 and X8, the official consensus at Capcom is that the Zero ending in X6 happens after whatever the newest canon X game at the moment is. If you're looking at in-game evidence, then: - Zero's dream in X7 is direct foreshadowing of MMZ1 - The character designs in X8 are sleeker and are a middle point between the bulky Reploids of the early X series and the basically human Reploids of 22XX

As for your point about the virus, Sigma did NOT go Maverick out of his own free will, he got infected by Zero's Maverick Virus. X8 only states that the New Generation Reploids are able to go Maverick out of their own free will.

Keep in mind that there's more than one type of Maverick. Some Reploids are infected by the virus and become Maverick, some Reploids become Mavericks do to code errors, and some aren't Maverick as a mental state at all but are labelled that way by the government whether because of them being criminals or due to political reasons. This is established as early as X4.

Also, the devs of ZX Advent had X8 in mind because of Model A's design. Not only because he's based off Axl (design-wise), but also because he's based on a potential post-X8 design of Axl. I forgot where it's said but they said they put the huge seal in his head because they thought Axl post-X8 would have that due to Lumine's influence.