I Finally Figured Out the Fox X-Men Continuity
OK, here's my attempt at making sense of it all - like it or not, I'm convinced this is what's going on. I'll try to make it as clear as possible; hear me out.
In my mind, the Fox X-Men film series depicts 2 different universes:
UNIVERSE 1: All the films with X-Men in the titles, as well as The Wolverine
UNIVERSE 2: The Deadpool films (which will be from now on abbreviated as DP) and Logan.
This explains why Wade is the same age in DP1 (set in 2016) as he is in X-Men Origins: Wolverine (set in the 1970s or 1980s). Logan's X-Men: Days of Future Past time-traveling to 1973 couldn't possibly have changed Wade's Origins birth date, which must've been in the 40s or 50s. This also explains why Colossus has an American accent in the X-Men films and a Russian accent in the DP films. Therefore, the DP films are a separate universe where Wade was born a few decades later and Colossus was raised in Russia.
This also explains why there are many young mutant children at the school in the final scene of DOFP (set in the year 2023), whereas no new mutant children have been born in 25 years in Logan (set in the year 2029). They are separate universes.
Within these 2 universes are multiple timelines, caused by the fact that each universe has a different methods of time-travel:
In UNIVERSE 1, Kitty Pryde's time-traveling of Bishop's and Logan's consciousnesses into their younger bodies in DOFP rewrites timelines.
In UNIVERSE 2, Cable's device in the DP films causes timeline-branching.
UNIVERSE 1 first has its timeline rewritten at the beginning of DOFP, when Kitty sends Bishop back. It is then more fully rewritten, starting in 1973, by the ending of DOFP. The last we see of the OG timeline is when the Sentinels are about to wipe out the last X-Men, then that reality is erased. The last we see of the new timeline, and that universe in general, is Logan waking up to the new, happier future - and that's the end of OG Logan's story.
UNIVERSE 2 is not rewritten, but branched several times.
It gets its 1st branch from Cable when he first time-travels; he comes from a point in time several decades after the events of Logan and after Laura is pruned by the TVA.
It then gets a 2nd branch when Cable saves Wade.
It then gets a 3rd branch when Wade saves Vanessa. During the events of DP&W, Wade is living permanently in this 3rd branch, where Vanessa never died. This is not the same branch as the events of Logan. However, all the branches are within the same universe - which is why the events of Logan cause the TVA's plans to affect Wade's reality. And this is why Laura is able to stay in Wade's reality at the end of DP&W without causing a paradox - she comes from the original timeline of this universe, then relocates to its 3rd branch.
Side-note 1: In the infinite multiverse, many universes are extremely similar with one key event that causes it to unfold differently than another. In the case of UNIVERSE 2, it clearly has similarities to UNIVERSE 1 - in Logan, Charles references the Statue of Liberty incident that happens in X-Men 1, indicating that the events of the OG trilogy in UNIVERSE 1 also happened in a similar way in UNIVERSE 2. But the difference between the two universes unfolds in how mutants are wiped out:
In UNIVERSE 1, Trask develops the Sentinels and then the timeline-overwrite causes mutants to finally be accepted.
In UNIVERSE 2, Transigen develops the mutant-suppressing poison which is mixed into widely-consumed food, impacting mutations and stopping mutants from being born after 2004. (This is why Russell is 14 years old in the year 2018 in DP2; he was one of the last mutants to be born in 2004 before the poison started suppressing the X-gene in pregnancies in this universe. This is also the cause of the Westchester Incident in Logan; Charles is not affected by a regular human disease, but rather by the mutant food poisoning that only exists in this universe that has made his mutation unstable, and is further inflamed by the emotional after-effects of Erik’s sudden death from said poisoning).
Side-note 2: Cable's device also enables universe-hopping, as seen when Wade briefly visits the MCU and talks with Happy, then goes home, in DP&W. Clearly this is an ability that was engineered within the 6 years that pass between DP2 and DP&W so that Wade could reach the MCU, and in general do some reality-traveling without continuing to affect his own reality; he certainly wouldn’t have wanted to risk any meddling that would’ve jeopardized Vanessa’s salvation in the branch he created.
Side-note 3: You could also view Wade killing Weapon XI at the end of DP2 as another bit of universe-hopping, and possibly creating a branch in UNIVERSE 1. Or, you could view it as an unserious meta-joke that doesn’t actually have timeline implications, just like the X-Men somehow appearing young - including McAvoy!Charles - in the year 2018 in DP2, which is flat-out impossible in both universes and clearly only a joke.