though the real issue seems to be that you’re just unwilling not to consistently give the movie the benefit of the doubt

I've given pretty detailed explanations of why the things you've brought up don't seem unrealistic to me.

The gas station scene is heavily contrived to provide an older-brother or father figure

We just seem to have radically different interpretations of what we saw on the screen. You seem to be very confident about what you think the filmmakers intended and how most audiences will interpret specific scenes, which I dispute. To my mind Erik's main purpose was for world building, to contrast the way the UK had regressed to an older type of civilisation while the rest of the world moved on. Erik's character is petulant and kind of obnoxious and Spike never really takes to him, I don't see him as a familial figure to Spike at all. If anything he presents another type of authority figure that Spike rejects and stands up to, in the scene with the child.

This is not only a big tonal departure from the previous movies, but also within the movie itself. What is this, an action, adventure, horror, buddy, sci fi, drama, comedy movie?

This is probably in line with a lot of the people I've talked to who had issues with the film, it didn't fit into their expectation or categorisation. I don't know why this is a negative for some people but each to their own. Danny Boyle has basically built his career on subverting categorisation and challenging stylistic norms.

For a large part of the movie, her illness just isn’t relevant to what she’s doing.

I find this kind of a baffling statement. Her character is defined by her illness, and it defines the relationships of the whole family to each other. Right from her introductory scene she is her illness, she is no longer in control of herself. Spike travels onto the mainland because of her illness. During her travel she is continually regressing into a childlike mental state, because of her illness, inverting the parental role with Spike. This is central to Spike's character arc, imo, rejecting the expectations and traditions of his society and choosing his own path, taking over the parental role from his mother and eventually taking complete ownership of his own life after she is gone.

And of course her death is central and necessary for the most overt theme in the whole film, the fleeting preciousness of life, whatever shape it takes.

Are you saying it isn’t heavily implied those are the same kids?

When the zombies burst into the room with the kids at the start, there is a very specific shot of blood splattering across the TV screen. To me that was a very intentional message that those kids are dead. There's definitely something going on with the blonde hair, but it seems to somehow be tied into the fact that they're all dressed very similarly to the celebrity and child molester Jimmy Saville, who had the exact hair that Jimmy's group all have. I don't know what's going on there but I don't think they're the same kids. I guess I could be wrong about this, all the kids at the start being blonde is certainly a bit confusing in the context. But if I am I'll wait and see what the explanation is before I decide whether its believable or not.

Isla wandering into the distance to be killed by a morphine blowdart which seconds earlier had only served to make Spike a little woozy - dumb.

He can't have different strength darts? Is that unrealistic? This seems like the key difference between how we've viewed the film. For me this is a minor detail that serves the greater plot, and can easily be explained without jarringly breaking suspension of disbelief, even though it isn't specifically explained. It didn't stand out to me at all. But if that detail jumps out at you and interrupts what the scene is trying to do, I guess that undermines the film for you.