Horror

I think Erik's purpose as world-building would be redundant. It was already achieved by Spike's dad pointing out the quarantine patrol. He could have served a world-building purpose if he helped explain why Britain alone is the quarantine zone and not France where 28 Weeks Later clearly depicts the virus spreading, but he doesn't.

I'm not opposed to tonal departures from previous films in a series, but when it plainly contrives justification to jump between genres mid-movie, to me this screams artistic compromise for the aim of broadening audience appeal. Especially combined with the technical choices like the 30-iPhone camera rigs, it feels less like they were trying to reframe the series and more like they were taking the piss, blinded by hubris, motivated by a payday, etc.

she is no longer in control of herself

I'm referring to the large parts of the movie where she clearly is. She alone has the presence of mind and body to endure danger to save baby Isla, to save Spike while he's sleeping. The way the movie depicts her, when they wake up and she has apparently forgotten what she'd done it's almost as though she's hiding the truth to mentally shield Spike. Following her diagnosis she even explains her previously unspoken awareness of her own confusion. She isn't continually regressing; she's intermittently regressing. She is more helpless at the start of the movie than at the point of her death.

I hadn't considered they were a depiction of Jimmy Saville, I think you're right. It would add to the backstory of the kids, given they are depicted as related yet socially distant from eachother. I'd imagine they were in a cult, probably half-siblings with Jimmy with the same crazy Catholic-molester-cult leader father, his bloodline carrying the mutation that makes them subservient to Jimmy, and his character which Jimmy emulates. The shot of the TV you're talking about is likely a red herring, not because of this theory of mine but because there's simply no reason for the group of kids to exist as they are depicted. It's almost certain that whole scene's purpose was to set up the next movie.

He can't have different strength darts?

Certainly he can. Again, I agree with you that suspension of disbelief is fine and normal in movies. The point at which it becomes bad is when a significant part of the narrative arc of a movie heavily depends on that suspension of disbelief. It is fine to assume that Ralph Fiennes' character has devised some way of surviving on the mainland because he is already built up as an expert survivalist, so the specific methods he used don't require extensive explanation. His medical expertise means it's even fine that he's somehow found a way to either synthesize morphine himself, or scavenge it. No critical part of the narrative arc of the movie relies on these facts. However Spike and his mother's acceptance of her death and the method of her death all hinges on that Ralph's morphine darts, the purpose of which is to temporarily sedate Alphas, are actually pre-prepared for mild sedation of a child, euthanisation of a human, and presumably a range of other purposes. Can you see how that would need some sort of surface-level explanation to be believable, or do you really think it's ok that we are just to presume that he's a master of adjusting bootleg morphine blowdart dosages on the fly? Perhaps if that were the only case where such a leap of faith is required by the audience to make sense of the plot, it wouldn't bother me.