Capturing room audio (audience questions/comments)
Audience questions : there's not much alternative than to have another mic (probably a hand-held one) ready to be used by the audience. If the room is really small, possibly a cardioid microphone on a stand pointed toward the audience could do the job.

Round table : have one or two hand-held mics that the speakers take when they want to speak. That's a constraint on them obviously. You can also try a boundary microphone on the table.

Notice also @stib good suggestion below to have a boom operator for audience questions.