There were some major plot elements that violate the rules of 'what if?' movies, and, if you give them too much thought, negated and conflicted with other plot elements. Still, it's only a movie, so best not to nit-pick the details too closely.
The script is quite clever in a number of details, which produced in the audience the expected chuckles. Interesting to have a main character that you both sympathized with (what a dilemma!) and regretted his fundamental dishonesty.
Ed Sheeran, Kate McKinnon, and other supporting players gave very good performances. Lots of laughs in those scenes.
The expected final scenes, of course, redeemed him and we assumed 'they lived happily ever after', since the film-makers decided, ultimately, that this is a Romantic Comedy, not an absorbing sci-fi puzzle, a la 'Inception'.
But, for people who were alive 1962-1970, it's all about Those Songs. During the film, they were all performed by the guy, but it was genuinely moving to have the original 'Hey Jude' play, in its entirety, over the closing credits. We stayed until the very last note.
I can't imagine viewing this film from the perspective of anyone born after 1972. How could they possibly understand what The Beatles meant to Boomers, where we lived, in Real Time, the whole astounding journey from 'Love Me Do' to 'Golden Slumbers'?
And now to deal with The Secret
Let's be clear - the scene makes no sense, given the basic premise of the film.
When the door to that farmhouse opened, I gasped, and then watched Karen gasp. The impact of that scene stayed with me for hours and, even now, the next morning, I feel it.
I am still filled with quiet gratitude that we have been given the sublime gift of a few minutes with our old friend, John.
It was a reminder that, although we lost something profound all those years ago, sometimes you get a chance to get back to where you once belonged.