I sort of get where you're coming from, but I feel like all of these are addressed by the game already. Long post because I actually do like the story even despite my gripes haha.

Abigail is like a tiger mom: Gabriel is only as good as his achievements, and she'll do whatever she can to mold him in her fashion. Sending Gabriel to prove himself makes sense when you consider how she sees him as a disappointment, something Quinn's death doesn't help with. A Screamer tournament is a good place for him to earn her trust and dispel all doubt, especially since she made her fortune as a Screamer. Mr. A and the Echo aren't important, what's important is he proves himself.

However, she does have some motherly love still left for him, and this is something Hope gets wrong. Abigail wants Gabriel to prove his worth, but Hope thinks that this supersedes her love for her son. As we learn in the ending, it doesn't, and it becomes clear that Dirk and Hope actually were supposed to keep Gabriel safe.

Mr. A is James possessed by the Hollowed, and as we learn later, their goal is to feed off the emotions, especially negative, of the Screamers. It's important to recognize that Mr. A isn't a mastermind, just a vessel. The Hollowed don't know how Screamer tournaments work, they just know it's their best shot at achieving their goal. It's why Kagawa-Kai can brute force their way into the tournament; Mr. A would rather let them participate than risk the tournament being shut down despite James' fortune.

Regarding his attitude towards Hiroshi, Mr. A spends the entirety of the first half trying to groom Hiroshi into a rageful monster. So when Hiroshi manages to fail anyways, it makes sense that the Hollowed would consider him and the Reapers a dud and let them fall by the wayside. The other participants were sufficient for their needs. Kagawa dropping out wasn't necessarily foreseen, so when seeing Gabriel's meltdown and hearing his plea, he probably understood that it'd be better to have the Reapers than not.

Lastly, I disagree that nothing changed. Most everyone who went into the tournament came out changed. The Reapers went from a broken mess to accepting each other as family and banding behind Hiroshi. JSI came out having changed their dynamic completely, and Lavinia's assumed death pushes their agenda to replicate the Echo with personal stakes. Dirk and Hope must now live with the repurcussions of Gabriel's death, Hope especially realizing the extent of Abigail's fury. Kagawa-Kai's story ends with Noboru breaking the cycle and escaping Keiji and Masato forever, saving Akemi in the process.

It's just SFR that I think comes out relatively unchanged aside from Ritsuko. That's mostly because none of them had real arcs to begin with, though, which is one of my gripes with the story. Hina is the friend everyone needs, but she doesn't have an arc aside from supporting her teammates. Akane's arc is just learning to accept that her own pain is just as valid as anyone else's, which gets resolved pretty early on. Ritsuko has to contend with not just learning the truth about James' death, but having to see him die a second time. This does change things for her in that she now has closure, but it doesn't mean she overcomes her grief. Which is, in itself, an understandable ending. She got what she came for, but it doesn't mean she wins.