I'm not entirely sure a creature is meant to gain one level from each scroll they study.
Otherwise you'd have at least some sarrukh and netherese liches and elven baelnorns persisting today with 50+ levels.
So your argument for it not working is because ... there'd be walking penis-extension NPC spellcasters running around? In
the Forgotten Realms? Have you even read
any of the setting material? Like, at all? We are literally talking about the Land of Walking Penis Extension DMPCs
TM, here.
Leaving all that aside, you're actually right. There totally
are ancient baelnorns living in Myth Drannor itself who are explicitly guarding the treasures, including the
nether scrolls, that lie within. There's also multiple wyrm dragons, a bunch of devils summoned by servitors of Bane, cabals of illithiliches, a host of phaerimm (who are about as broken-retarded as the sharn, only they want to drain all the life-energy out of the
planet) and some dark cult. The entire fallen city is literally a hive of hostile factions all warring with each other and with intrepid adventurers over the treasures of the city.
Snark aside, actual rules text ahoy!
Reading even one nether scroll offers considerable insight into the Art. Any character studying one immediately gains one level in an arcane spellcasting class of her choice ... A character who manages to read all ten scrolls that make up a chapter gains an additional benefit whose nature depends on the topic studied.
That's straight out the book with the exception of the bolding. Reading one scroll gets you a level. It's not one or more scrolls, it's one. So when you read a chapter, you also gain
ten levels in whatever arcane class(es) you want.