Given that questions are submitted so far in advance for webchats, and whoever is answering them gets a chance to look at them and prepare answers, I think it would be useful if MN provided a space where all the questions were organised under different headings.
So, people could ask what they liked before a pre-arranged deadline, giving plenty of time for mods to go through them, remove any that were directly offensive, and also word for word repeats (though maybe the number of people who asked the same thing could be recorded beside the question) The rest could be arranged in topics - in this case for example, maybe general Labour Party questions, GRA-related stuff, leadership election, personal beliefs and values, fun stuff about biscuits. (And perhaps a portion of time set aside for spontaneous questions and responses.)
This way, politicians (or whoever) could choose a representative few questions from the list. Everyone could see the ones they'd swerved (and draw their own conclusions.) They could maybe widen their answers to address more points.
Crucially, the politicians could also gauge the level of interest in a particular topic, and popular feeling on it. If there were 200 questions about the GRA and only 7 on hobnobs vs chocolate digestives, that in itself sends a powerful message.
No experience of organising web chats, so am prepared to be told this wouldn't be possible. But it strikes me as being a more democratic and productive approach.