Sadly, I don’t think Teen Wolf is as good at representation as you think it is. Also, this “All or Nothing” business (I presume you mean in fandom?) isn’t really A Thing, as far as I know. Yes, there’s a lot of criticism over many aspects of Season 3, but that doesn’t mean people suddenly hate the entire show or are arguing with other fans who think that Allison and Lydia are strong characters. It just means they’re disappointed in the direction the show appears to be taking.
With regards to Dethan and Allison & Lydia, I’ve never really got the impression that people are forgetting them? Like, Jeff Davis has done a pretty good job of making it clear that the world of TW is, as promised, homophobia-free, and a lot of that is down to the way Danny is treated by other characters. With Allison & Lydia, they’re great characters, although I don’t think “lack of shame about being sexual but not submissive” is really that big a deal, since it is NOT DIFFICULT to find sexually active girls and women on TV in 2013. Honestly I’d be more surprised if a teen drama DIDN’T show teenage girls having a whole bunch of sex. It is good that Allison and Scott are shown to respect each other and practise safe sex, though, which is something that you don’t see as much of in shows like Gossip Girl. Also, Teen Wolf’s general lack of the male gaze. :)
Anyhow, in a more general sense, you can feel however you want about it, but IMO it is TOTALLY VALID for other people to be annoyed by the female and POC deaths in Teen Wolf. Allison, Lydia, and Melissa are all great female characters, but women are still vastly outnumbered by men on this show, even if you include side-characters. Also, your list of female and POC characters really is just a list. They exist, but side-characters like Kali and Morrell have received a fraction of the meaningful screentime or character development of people like Deucalion, the Twins, or even Gerard Argent.
(Oh, and while we’re on the topic of Gerard, can I just mention how utterly terrible that flashback episode was? An ENTIRE EPISODE was dedicated to backstory that mostly wasn’t even necessary to the series as a whole. Learning about Deucalion’s background was useful, but that could’ve been done in 5 minutes. Aside from that, all we learned was: Gerard and Peter are amoral liars, and Derek Hale has had a tragic life. WHICH WE ALREADY KNEW. Plus, a female character was invented and immediately killed off just to add to Derek’s tragic backstory manpain, which was ALSO completely unnecessary because he already had a very satisfying tragic backstory, which was partially negated by this stupid new additional tragic backstory. Arghh.)
If you look at the characters who were added and subtracted this season, the results aren’t great. Jennifer and the Twins had relatively similar roles: seduce and destroy. However, Jennifer (who was on a mission of revenge after her friends were killed and her face was mutilated) was not “redeemed” by her feelings for Derek, while the Twins (who killed people while “just following orders”, including Boyd), already seem to have been redeemed without really expressing any kind of regret that they’d murdered people or helped Deucalion on his evil quest. Both have been ~saved by the power of love, despite the fact that Aiden was portrayed as being significantly less conflicted than Ethan about what they were doing, AND the Aiden/Lydia relationship was mostly shown to be them hooking up/using each other, with a SUDDEN BURST of ~emotions~ when he was literally dead and lying on a table in the vetinary clinic. In the final episode, everyone cared more about the ongoing survival of Ethan and Aiden, who murdered Boyd, than they cared about Boyd’s actual death. Lydia actually SAW them kill Boyd, but still dated Aiden, because of… reasons…???
Ms Morrel’s role is unclear (and not in a good, “ooh what a mystery” kinda way), and she mostly appears for exposition reasons — much like Deaton. She had a really cool moment with the circle of mountain ash, but compared to the 20 times we’ve now seen the Twins superglue their torsos together, this isn’t great representation. There are so many characters on the show that most people probably finished the midseason finale without realising that we don’t even know if Ms Morrell is alive or dead.
Meanwhile, Jennifer and Kali are both dead, while Deucalion and Gerard are still alive to be brought back whenever the show needs a spare villain. You write that Ms Morrell and Kali are “strong characters” but they are only strong in the sense that we see them doing “tough” things. They’re not strongly-written, in the way that Derek, Chris, the Sheriff, Deucalion, or the main teen characters are. And even Deaton, who now receives quite a lot of screentime, is usually relegated to exposition scenes and being ~mysterious~ rather than growing into a Giles-like role, or becoming a more ambiguous character. Aside from Scott, POC characters in Teen Wolf NEVER receive the same quality of character development or screentime as white characters.
While S3 included several awesome women (Lydia, Allison, Melissa), there are plenty who are poorly-explained ciphers (Cora, Ms Morrell), receive very little meaningful screentime (Kali, Ms Morrell again), or have been killed off (Erica, Kali, Jennifer). TBH, S3 is not that much better than most of the mainstream TV shows that Tumblr fans tend to avoid. Yes, there’s a gay guy in the secondary cast of the show, but it’s still the case that white male characters get a zillion second chances while Boyd, Kali, Jennifer and Erica were all killed off with relatively few regrets. This isn’t some kind of weird internal fandom war between people who appreciate “strong characters” like the ones you listed above, and people who ~hate~ the show for single-minded Tumblr Social Justice reasons. It’s fans getting pissed off because they were promised a show that was better at gender/sexuality/racial representation than things like Supernatural, but were let down on that promise.