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.