On the short list of 'big questions' with the case for sure. If he didn't know any of what was about to happen, why be scared or leave? No witnesses saw Eric talk to Brooks as far as I'm aware, Brooks had the reputation of a liar, and it just seems like something out of a script.
I'd be curious if there is an account with him mentioning Eric not wearing his hat or parking in the wrong parking lot before No Easy Answers, for those seem like the details he would get wrong if he were lying.
I do figure it happened, but have always been very skeptical about it from the first day I heard it. I'm not sure that Brooks had A lunch. Brooks seemed to say he was already thinking about skipping his next class, and Eric telling him to leave gave him the excuse. Even if you were skipping class, do you "go home" or start walking down the street, rather than just hang out in a part of the school away from your next class, like the commons? Was he skipping the day rather than just a class? It doesn't seem like he was coming back.
I've seen some speculate that Brooks, Chris, and maybe others were in on the plot, and then once the bombs failed chickened out and only Eric and Dylan remained. Pretty crazy if you ask me, especially with my skepticism towards the whole "they realized the bombs failed" narrative. Though, it is interesting how many escaped running out the north entrance at the back of the school, and had they recruited a third that seems the place to put him. Then again, I think entering the west entrance and cutting people off did that job, and Gardner mixed with their inaccurate shots may have stopped that from happening. Hard to believe the usual "Sanders told them to run the other way, thank god the gunmen never thought of that," especially with the shooting starting on the stairs and Dylan's reaction to seeing people running the other way with no bombs exploding seeming to be satisfaction and even "woohoo" when meeting Eric back on the steps.
As an aside, in the Basement Tapes they say "Chris pizza's place" is where they kept the napalm, and Chris says it was him, though I think their boss was also a Chris, and he says he refused to keep it at his place but they don't seem to act like he refused. Then with the final goodbye Eric calls him Morris. Was he called Chris or Morris? Maybe Dylan called him Chris and Eric Morris, but that's a bit odd. And of course if he was in on the plot presumably they wouldn't be saying "if you live you can have my stuff".