Interesting theory. Just one question.
Given that Picard has a sincere wish to leave the Nexus, why would it want to fool him and keep him in? Does it have an ulterior motive?
No one can really answer that question, because no one (not even the scriptwriters) knew exactly what the Nexus was, other than it being a plot device to get Picard and Kirk together. We hear it described one way, and see it act exactly the opposite way. And I doubt the writers gave much more thought to it than that.
The theory that Picard is still in the Nexus is based on two premises: that Guinan's description of it was accurate, and Picard's actions after he "leaves" it. Guinan flat-out states that you won't want to leave the Nexus once you're there, and you won't care about anything else going on in the real world. This is the exact opposite of what we see when Picard is there. So either Guinan is wrong, or Picard is being fooled by the Nexus.
So let's examine what happens with Picard in the Nexus. Because his family line is foremost on his mind, that's his initial fantasy when he first arrives. But as soon as he sees Rene, he's still thinking rationally and knows Rene is dead, and the fantasy is broken. He realizes that he needs to get back to stop Soran, and that wish then becomes foremost in his mind. So then the Nexus gives him the fantasy that he's going back to the real world to stop Soran by creating a fake "echo" of Guinan to manipulate Picard into thinking he can return to real life with some mumbo jumbo about the Nexus being able to return him to any point in time (something that the real Guinan never said.) She then has him team up with Kirk, who "from his point of view, just got there himself." Hold on now. What?
Kirk died 78 years before. If he really got sucked into the Nexus, then what has he been doing for all that time before Picard found him? Why did he feel like he "just got there?" Sure, we can chalk it up to the "mysterious" properties of the Nexus, but that's just a bullshit answer (as I'm sure the scriptwriters thought and just as easily ignored.) So we see "Kirk" just being curmudgeonly about his "time being over" and Picard grousing about making a difference "one last time" ...as if Kirk is secretly Picard's hero and that Picard also wants to have the
torch passed to him...etc., etc.
And if Kirk really was there, then his fantasy makes no sense. It was clear from the first scene of the movie that Kirk's greatest desire was to get out of retirement and command a ship again. So why is his Nexus fantasy...being in retirement? Perhaps because in Picard's mind, Kirk should
stay retired? Hmmm.
So they leave the Nexus, choose the worst possible point to arrive back (but one with the most potential for action and danger), they defeat Soran, Kirk dies, Picard buries him under rocks at the top of the mountain (again, what?), walks back to the Enterprise, doesn't seem to be bothered in the slightest that his beloved ship is destroyed, casually tosses aside one of the rarest artifacts in the galaxy just to retrieve his photo book, and then spends the next three movies kicking Borg ass, taking up arms against the Federation, and meeting his asshole younger self so that he can kill him too.