Note, I said one round, not FIRST round.
And, since we're insisting on being pedantic -there's no such thing as "core" in 2e. Everything in 2e was "core". Thus the Fighter's Handbook was core, making that -2/-4 for two weapon fighting largely moot since you could spend a couple of proficiencies to get rid of that.
You keep insisting on 3d6 for character generation, but, forget that 3e assumes elite array for judging PC's - 15, 14, 13, 12, 10, 8. A 25 point buy value character. That's what CR assumes anyway. So, suddenly your Con bonuses are largely limited to +1-+2/level. Not exactly going to make much difference when 3e monsters generally do between twice and six times more damage than their 2e counterparts.
I'm still rather baffled why you think that 3e characters hit more often. Monsters in 2e are typically around AC 5. And limited to AC -10. You want to talk dragons? How about that Elder Wyrm 3e dragon with a 35+ AC with Shield and Mage Armor on top. Oh, right, the fact that every 3e dragon is ALSO a spell caster doesn't matter I guess.
IME, a 2e party was effectively one or two levels higher value than a 1e party - if the 1e module was for, say, 5th level PC's, I could use 4th level 2e PC's and expect them to do quite well. Watch what a 2e party does to Keep on the Borderlands if you think that 2e parties weren't MASSIVELY more powerful than their Basic D&D counterparts.
Oh, and you want to bring up multiclass characters? Hrm... my 3e fighter/wizard has 2 levels of fighter and 3 levels of wizard, while, if the party is 5th level, my F/MU is 4/4 - yeah, so much weaker than the 3e version. To the point where no one ever took multiclassing other than a couple level dip if you were a caster because it so badly weakened your character. A two class character in 2e is maybe (and only maybe) 1 level behind the rest of the party. Even with three classes, you're generally only one level back. Woo, I lose one level of fighter to gain FOUR levels of Wizard and Five levels of Thief. Gee, that's a hard decision.
Yeah, it looks like Lowkey was right. You aren't looking for discussion, you're just looking for validation. Well, no. 2e is by far not the most lethal edition. Third in line, maybe. Depending more on how much Fudge was being played during the game. But most lethal? Not even close.