The Jean Grey Thread

1- Xavier had blocked the vast majority of her power, so she just displayed limited telepathy and telekinesis. That was it. She was at a very early "Marvel Girl" stage at that point. Xavier was seriously freaked out by her "Phoenix" nature and level.
Cyclops and Storm were powerful and skilled, but just "rusty" and "out of shape" (from a conceptual point of view). They had stopped training themselves long time before X1.
In X2 (six months later), they were fine.

2- The continuity is super-okay. Deadpool 3 will be a metafictional superhero comedy, you cannot take that as canon (unfortunately). It's an alternate universe.

3- Famke was very fascinating. Sophie was more vulnerable and melodramatic. I think that the XCM Jean Grey was a constrained person, very afraid of herself, and that's why she (they) looked so distant and emotionally detached. You can see the change of Sophie's Jean at the end of Apocalypse and during the entire Dark Phoenix.
Famke was marvellous in both X2 and TLS.

 

Again, the first X-Men movie was written years before movies like TLS, FC, and DOFP were even conceptualized by the filmmakers. I'm looking at the first film as it was made in 2000, prior to the writers expanding, and explaining the mythology in subsequent sequels, prequels, and requels.

Depends. An effort is being made to fit the movie within the Multiversal framework established by the MCU, do the Fox films by extension, will also be explored through that framework. So retroactive continuity errors -- like FC clearly being written as a reboot of the mythology before the studio backtracked, with ensuing continuity errors as a result of trying it make it fit -- will be explained as it being an alt timeline from the OT.

I simply don't agree. Sophie's "emotional distance" felt more like a weakness of performance, than a perk, especially in Apocalypse, as the character wasn't written to be emotionally closed off at that stage in her life. For Famke, it makes more sense, hence why it's to a lesser extent, because it feels more like a creative choice. But again, it's a far cry from the source material, and makes Jean feel very "alien"; she lacks the warmth, and emotional vulnerability of classic Jean.

Honestly, the same goes for Marsden and Halle. They all felt very "muted" in those movies, but it felt like a directional choice on Singer's plate

 

I hope they cast late 20s to early 30s for Jean. I don't want a teen / early 20s Jean again.

 

You're right. ANYWAY, the 2000 movie was written and filmed in that way, so we must "fill the gaps" by ourselves. And it works beautifully.
By the time the events of X2 occurred (six or eight months later), the team looked well-trained again.
That said, during the final confrontation with Magneto in X1, Cyclops, Storm, Jean Grey and Wolverine worked as one single unit. They did a fantastic job in taking down the enemy, synching their individual powers to each other.

The continuity errors between the Original Trilogy/Origins and First Class are tiny. We can fill the gaps by ourselves.
Read this, for instance: Apocalypse - 5 facts you didn't know about Xavier...

In Apocalypse, she was scared off by herself. She didn't want to hurt anyone, she was trying to assert her control over her nature and (infinite) powers. She was a very troubled person. She became Phoenix at the very end, and it was the most astonishing and epic moment of the movie. Sophie truly delivered the best.
Dark Phoenix was an entire movie focused on her, and she did a great job with it. I'm sure that the movie will be revalued within a decade. It truly deserves that, because it's a very fascinating, exciting and deep superhero movie.