Unfortunately Dear The Jester you're far from the reality.
Whenever an encounter called for an NPC party (which was a fairly common occurence) the DM had to create a group from the ground up using PC rules. No escape from that. Same thing in the 2nd edition. It became quite tedious in the 3.xEd era where the number bloating was at its maximum. The occasionnal non PC class such as the berserker or neanderthal were exceptions, not the norm. Only in 4ed did we see set stat block for pc class related NPCs which did not involved hours of preps.
Drow in the fiend folio:
no appearing: 5-50
AC: 4 or better
Move: 12" (15"females)
HD: 2 and better
No of att: 1 or 2
bla bla bla
If a drow party of more than 10 is encounter, in addition to the 3rd to 7th level fighter (do the stat yourself here) there will be an additional fighter/magic user of 3rd level (do the stat yourself). If 20 are encountered a female fighter cleric of no less than 6th level (do the stat yourself again...) will be there. And it goes on and on and on up to 50 drows encountered where in groups higher than 16 will always be at least 50% females.
Go watch the svirfnibli, same. Dwarves, same, Elves? Same again. Same thing in the 2nd edition. Yes some monsters had a stat block. But they were not the norm.
In fairness the character classes were way less complicated in general. For a fighter you boosted HP, saves, and THAC0 and you were done. It wasn't that different that just ad hoc adding the numbers, but the designers for the most part just pulled level values from their butts and called it a day.
However, from a transparency standpoint nothing stopped you from saying: AC 2, Hp 52, THAC0 10, Saves vs stupid list of crap 14, Damage 1d8+3 twice per round. That was an NPC for AD&D, super, super simple. It isn't like the player would honestly know any different in the end. You can do that same thing in 3.x, but the expectation is that you don't because nothing is setup as completely arbitrary numbers.
3.x really went to town the idea that say a beholder had to have X "levels" in the abberation "monster class" so it could have a +x Will save, a +y Intimidation skill, and +z to hit. While the exact values didn't match concept applied mirrored building player characters. Where as if we look at AD&D it basically went with "here's a bunch of numbers, it's a beholder."
Nothing is wrong with either approach, but effectively saying NPCs follow the character creation rules is annoying, because I'm not creating a player character as the DM I'm creating a challenge for the players to over come. Sometimes I want to make things mirror the players characters, sometimes I don't. The numerical values and "abilities" I give that challenge don't really matter from a game perspective, because in the end you can justify and rationalize anything however you want.
As a GM if I did the double concentration I would explain it thus, "Want to concentrate on two spells at once? Super. this crazy wizard can do it, but learning how made him insane. He also didn't write it done, because he was insane and paranoid. So, good luck you go spend the next 20 or 30 years learning that same skill, and also end up insane. I'll take that character sheet and it is now an NPC. Oh, no, you changed your, sounds good to me."