A perfect case in point is the Space Shuttle tragedy. I can personally give you the names of well respected educators who will say, under oath, that they saw a surface to air missle take down the Challenger. Did it happen? I don't know. I do know that the overwhelming evidence is that it didn't.
As this relates to the Columbine tragedy, we realize we have numerous witnesses who have made statements that contradict the official story. We also have even more who do not. We have limited video evidence that supports the official story. We also have writings by one of the known killers which seem to support the official story. The eye witnesses are the least reliable of all the pieces of information because they were in a highly stressful environment (gross understatement) and were probably more concerned with their own survival than with making a mental record of who the shooters were and how many there were.
My gut instinct is that there were 2 shooters, but they had shared their plans, at least to a limited degree, with others. It seems clear that there were people who knew the kids were making bombs. There were kids who knew they were buying guns and ammunition in quantities inconsistent with normal teen behavior. There were also kids who knew of the resentment they was feeling towards those society would have called either normal or exceptional students and athletes. To me it is highly unlikely that there were not at least kids who knew what the killers intended to do. I feel some truly thought they were bluffing. Others had varying degrees of belief that it would happen. I also think it is likely that there was one or more person who knew what was going to happen, and when. That person or persons likely was involved in helping to transport the devices that were intended to be used as bombs.
I don't believe for a second that these two kept a plan of this magnitude to themselves for a year. Harris was clearly obsessed with sex and trying to impress girls in an effort to bed them. I would expect that he might have felt that the "bravery" (in his mind, not mine) that he was exhibiting by formulating and carrying out this plan would be impressive and could be used as a tool to that end.