That's because Revan broke into big Sith bases five times (Taris, Manaan, Korriban, Rakata base and the Star Forge). Not every Jedi Master does this, especially one as famed as Vrook who himself admits he couldn't do the quest. The only other time Revan encounters Dark Jedi is when he's ambushed by three, because Malak specifically hunted him. He likely sent DJ after Vrook too, but not when he was safe on Dantooine. Bastila remarks that Dantooine was relatively safe of Sith attacks.
You ignore that Revan and Malak waged war against the Republic. When dark Jedi show up... who ya gonna call... Jedi! Vrook was a warlord, he was the defacto leader of the Jedi council, he was chosen over Kavar to have full authority during a war time situation. Zez Kai I admit is the weakest of the three but he nonethelss saw more combat than someone like Kit Fisto. Do you assume that the Jedi council sat on their asses when the two Lord's of the Sith appeared from the outer rim? Kavar led a team against Malak. Vrook we can only assume is stronger from what we see given the context, as well as his resisting Traya better than the other three.
I believe the number was quantified to 20. Jolee was either off his knocker or exaggerating. There were about as much Dark Jedi in the PT times.
As I recall form the comics, Exar Kun appeared before the galactic senate and slaughtered the senator. It seems according to KOTOR, his words left an impression on many of the Jedi. This was no minor event, it led to a major Jedi Civil war. One that was subsequently followed due to Malak and Revan.
Knowledge is also gained through time as well. A lot of knowledge was lost during the Civil War (because many Jedi died) but most of it was preserved in the archives on safe little Coruscant (which have, to my knowledge, remained untouched). You could make a far better argument for Sith knowledge though
True, but much more was lost in comparison. So much was lost from Ossus. Kun made sure that the Jedi would not have the advantage of ancient lore. The order was devastated after Nihilus and Sion. Many of the ancient holocrons were lost. Also a lot of ancient Sithknowledge was effectively lost forever.