The statted maug is wielding a two bladed sword with a +5 to hit. Thus he must have exotic weapon proficiency.
The stat block doesn't take into account the -1 to hit for a large creature at all.
The statted maug can't use his slam as part of a full attack
You are right about the slam having an extra +1 to-hit. Can't believe I never noticed that.
For the 2-bladed sword, I would rule they are proficient with all martial weapons, and that 2-bladed swords are treated as martial weapons for them. Ditto for heavy armour.
I think the designer was thinking that as an npc to be fought, such minor details weren't worth mentioning or were simply glossed over. Why would the PCs care that he doesn't have exotic weapon prof anyways?
All I can say is that he was ahead of his time, because that is the exact design philosophy for 4e. Simply cook up the numbers and cheer over not having to justify them in-game.
As for horror stories, just about every splatbook has npcs with stat errors. One of the more extreme ones I recall was in exemplars of evil, where the ancient blue dragon npc is missing +8 to-hit on all her attacks.
I recall the cormyr line of adventure books was extremely riddled with such mistakes. It didn't help that I wasn't used to the new statblock at all, so it took a lot of time for me to reverse-engineer their stats to double-check.