Certainly, FDA makes errors, but the reports prior to ADCOM meetings are publicly issued and instantly overanalyzed by the companies involved and investors alike. They go through cycles of review before publishing, but of course that doesn't mean they are 100% guaranteed to be correct.
The ADCOM meeting starts with FDA talking about their review of the file, and then the company can bring in experts of their own to debate the fine points. Then the ADCOM makes their recommendation.
As for why can't the panel consider combined drug use the easy answer is that is not what they are there for. The name says it all, "Advisory Committee", and the committee gives advice on the questions that FDA puts before them. Like any consultant, they do what the client asks of them, but no more, and if FDA wants to make a decision that runs counter to the advice . . . well, clients have been doing that for years. Ask any consultant.