Okay, so imo Salamence deserves S. Since my explanation in the previous post maybe was kind of lackluster, I will explain why properly now, and why some of your arguments for why Kang is surperior compared to Mence are lackluster. First of, yes, Mence has solid checks that deal with all its sets. So is the deal with Kangaskhan, and every other Pokemon in existense. Ghosts with WoW are not the only things that beat Kangaskhan. Other things with WoW, Arcanine, Rotom-whatever beat it. Aegislash (sub more easily, but Wide Guard can in a 1v1 go for PP stalling out Sucker Punches with Wide Guard. Terrakion is a surefire way to beat Kang, and Lando-T with Superpower OHKO's most of the time, with Band you got it every time. You can't ignore the fact that there are many ways to beat Kang. Bulky waters are not reliable, but when the Scald burn comes, it stinks for Kang. Altough it has PuP, Intimidate is still not straight up ignored. Landorus-T is the most common Pokemon after Kang, and more popular on Showdown iirc, and is the best way to shuffle Intimidate. It can come in, Intimidate, U-Turn out, and come back to Intimidate once more, and not really fear the hit it switches into. My point here is that Ghosts with WoW are not the only ways to beat Kang, and there are several ways to beat Mence, and many ways to beat all Pokemon.Rotom (especially H), thundurus and zapdos will always check pretty easily salamence and then proceed to OHKO or cripple it then there are the others you mentioned. Tbh i don't really think salamence deserves to be S, imho people are just too paranoid when facing him.
Kang on the other hand is surpringly random. You can't never know if he has fake out or protect, low kick or PuP, return or DE (sometimes even ice punch instead of sucker punch). Every move she can have forces you to play differently (if she has PuP and setup you basically won/lost the game). Basically only ghost with WoW are a good way to stop Kanga
Now more about Mega Salamence. Earlier on I was kind of on the fence about it for S, but now I do feel like it deserves it. First of, although Japan Sand has made everyone use special or mixed Mence, physical is still highly viable. Mence+Clefairy was the original reason for why it was put in S. Clefairy's redirection combined with Friend Guardd letting Mence tank Rock Slide's like it is nothing, gives Mence an excellent opportunity to set up a D-Dance. When it has set up, Mence will kill something. It just will, okay, deal with it. Mence suffers from Hyper Voice spam, but Jolly 252 Atk OHKO's 252 HP 0 Def Sylveon with Double-Edge. In a situation where you have Clef+Mence and the opponent has Sylveon+something else, Mence can OHKO Sylveon and Clef redirects whatever comes form the other foe. Bulky waters like Suicune with Ice Beam is a big problem, that Mence needs like 3 D-Dances to deal with. However, as said everything has its checks. and something else on your team can deal with that. If you don't have a teammate that deals with bulky waters you have a bad team. Mence has the special set also, which is very good for Hyper Voice spam, and is indeed best paired up with Japan Sand. I honestly think that looking at the description of S rank, Mega Salamence fits the bill. It can perform multiple roles with tremendous effect (can Kang even do this?), has few flaws (weaknesses is a bummer though), its flaws can be taken care of with little to no team support (Clefairy is not a lot of team support, although special Mence needs a bit).
I will admit that Kang also has a surprise factor with Protect, PuP, Ice Punch and what not, it's type coverage is mostly the same (except Ice Punch). The thing is that almost no matter what what Kang runs, the same stuff walls it (unless you go to the level of Crunch and other elemental punches). As Synral said, different types of Mence has different checks and counters, but some Pokemon do wall all sets. The thing is that if you run some kind of mixed set, you might be able to lure in what would normally be a counter against either physical or special, and proceed to knock it out. Kang has a surprise factor to it, but so has Mence, having many different combos for coverage to mix up with on a mixed set, and also for pure physical or special as well.
(So this just popped up while I wrote my long ass post, lets make it longer) Tell me, how does Kangakshan beat Terrakion? Terrakion will always outspeed and OHKO with Close Combat. Other WoW users like Arcanine and Rotom formes do get knocked out by Double-Edge after first tanking a hit and Wisping it. Then they have done their job. Kang has taken a ton of recoil from Double-Edge, and is crippled and useless for the rest of the match. Aegislash is only beaten by Crunch Mega Kangaskhan. Mega Kangaskhan is also always walled by something very common.What i meant with ghosts with WoW mean that they are the only one that will totally wall kanga, the others you mentioned will still take a beating.
Salamence is always walled by something (usually very common), depending on what he runs. Kangaskhan has almost always ways to beat her counters.
And don't forget the neat fact of breaking sashes, an incredibly good thing that few pokemons have.