I have no problem with either the % to hit or the knowledge of resistances / vulnerabilities / remaining hitpoints. You have a party of several knowledgeable people, it's conceivable to say that they pooled their collective knowledge on these things at camp one night. Failing that, I have no problem with the "because tadpole" explanation, because it actually does do a fair job of explaining this.
As far as % to hit goes, just pretend the DM is giving you a penalty to hit because of the terrain / obstacles. It works out exactly the same, you just aren't seeing the attack roll.
I mean, the party isn't really that knowledgeable when you consider their backstories. Lae'zel has probably been isolated in Gith-world; Shadowheart doesn't remember a bunch; Tav is blank; and Astarion has lived as a slave for the past hundred years. Gale and Wyll are probably the most knowledgeable, and to me Wyll seems like he exaggerates his skills and Gale seems utterly absorbed with magic so it's reasonable he doesn't know much about goblins/etc.
Plus, in the full game release you'll be able to create an entire party of custom characters with 8 intelligence, 8 wisdom, no skills in nature or arcana, and all with the hermit/acolyte/urchin background. They will know exactly as much (everything) about the enemies as a party full of super-intelligent, super-wise rangers/scholars. I'd like it if nature and arcana checks, and just the Intelligence stat in general, were more rewarded.
I suppose it's possible that the tadpole is giving us specific knowledge about all enemies we encounter, but if so I'd prefer that it is presented in game. I don't remember any evidence of it giving us knowledge other than through dreams and telepathy...