You're isolating the numbers too hard, obviously no set is going to work without a little bit of planning or setup, that's how this game works. Obviously Malamar is going to need help while performing his job, but that's what the other five pokemon on the team are for. As such, he'll need help from hazards, such as Stealth Rocks, to lower the opponent's HP into 1-2 shot ranges for his Superpower to finish off and maybe even Sticky Web to allow him to outspeed some things. Also, softening up the opposing team is a good idea for any of Malamar's sets really, but with an Assault Vest set you can afford to take a hit or two if need be, hence the usage of AV.
Also, I believe that AV was introduced for more than just ramping up a walls defenses, it can be used to give artificial bulk to pokemon that would otherwise be quite fragile, like Malamar, turning OHKO's into 2HKO's and 2HKO's into 3HKO's, and so forth.
Well obviously, but the same can be said for other pokemon. If Scizor takes a Fire attack, he's dead, no question, but that's why you can switch out in this game. Like I said, you're isolating Malamar too hard, making it seem like he has to deal with every threat in the game by himself, but that's simply not true. If one was to use Malamar, you'd have to take appropriate measures to ensure his safety and ensure at least a 1:1 exchange for it's KO.
It's not because I isolates him, it's the Pokemon who makes itself being isolated like that.
No resistance. Again. Without any stats boost, how much damage can this thing do without a Band? There are not many Pokemons who are afraid of this Malamar without boosting you know. I have met him sometimes before, and a Bulky Set is worth 2 flamethrower from my Charizard Y, and 2 TBolt from my Specs Magnezone. The set which invests in attack can survive 1 only. Always a 1HKO.
How can you calculate for a pokemon which is almost always 1HKO or 2HKO by ALMOST-ANY-MOVES-CARRIED-BY-A-SWEEPER? And the worst part is he cannot do the same thing back. 2HKO from a Malamar without super effectiveness is something which is really luxurious for me to see, because it rarely happens.
And your example about Scizor taking a Fire attack is quite irrelevant here. Malamar's Superpower, which is the main reason why he is being discussed in the first place with his Contrary, is not effective against Fairies which there are a tons out there (Klefki, Sylveon, Azumarill) and is immuned by Ghost. I remember someone said that if he could sum up this metagame in 2 words, he would go with "priorities" and "ghost". What the hell does he do when the meta-game is already against him? Being 4x weak to U-turn and I think every U-turn users out there in OU is faster than him, is the end for this Pokemon. Play Rough Azumarill outspeed this Malamar, I don't even need to mention Sylveon. While Scizor can safely dodge a lot of threats using his U-turn, this one cannot since he is too slow.
TL;DR: His stats are too low for a sweeper. Sacrificing EVs and put them in Speed makes him 1HKO to almost anything therefore he can only accept that base 73 speed. He is outspeed by so many other threats. He is resisted to NOTHING. He needs too many turns to setup and he is not reliable at all.
Here are some stats for you to see how weak it is without boost in the first turn (because there is probably no 2nd turn for this Malamar):
Malamar Adamant Assault Vest:
Vs. Banded Scizor: 252+ Atk (custom) Superpower vs. 248 HP / 0 Def Scizor: 113-134 (32.94 - 39.06%) -- 99.93% chance to 3HKO
Vs. SD Breloom: 252+ Atk (custom) Superpower vs. 4 HP / 0 Def Breloom: 136-161 (51.9 - 61.45%) -- guaranteed 2HKO
Vs. Scarfed Rotom-W (not even Bulky Set): 252+ Atk (custom) Superpower vs. 4 HP / 0 Def Rotom-W: 107-127 (44.21 - 52.47%) -- 19.92% chance to 2HKO
Vs. Spec Magnezone: 252+ Atk (custom) Superpower vs. 172 HP / 0 Def Magnezone: 202-238 (62.34 - 73.45%) -- guaranteed 2HKO
Meanwhile:
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 252 HP / 252 Def (custom): 157-186 (45.5 - 53.91%) -- 43.36% chance to 2HKO
+1 252+ Atk Life Orb Technician Breloom Bullet Seed (3 hits) vs. 252 HP / 252 Def (custom): 378-447 (109.56 - 129.56%) -- guaranteed OHKO
252+ SpA Choice Specs Magnezone Thunderbolt vs. +1 252 HP / 252 SpD (custom): 186-219 (53.91 - 63.47%) -- guaranteed 2HKO
And remember that he always goes second with this kind of EVs spread so yeah.