This may be a bit late, but I'm a early-stage scholar in a management field that reviews similar proposals for our major conference. Here is how I approach reviewing symposiums (in no particular order):
1- I review them holistically (i.e. as one document), because that is how the final decision is made (accept or reject the entire proposed symposium). To answer your question, you don't need a separate review for each paper and for the introduction. Just write one review of the proposed symposium.
2- I structure my reviews similarly to how I review papers (summarize main points, strengths, areas for improvement, overall recommendation). I also treat them the same in terms of norms (conversational, developmental, includes what the proposed symposium does well, etc.)
3- I also keep in mind that conference papers are qualitatively different than published papers --- and are supposed to in our field! Conferences are where we get feedback on our ideas, get new ideas from conversations and presentations, and, above all, for everyone to learn :-) In short, I'm much more generous and err toward inclusion and look for the golden nugget of a great idea that needs to learn how to shine to guide me in terms of acceptance.
4- My advisor gave me these guiding questions as well to use in my review and these helped me out a great deal:
Does the symposium look interesting? Are the arguments clear? Do the presentations fit together well? Do you think there would be an audience for them? Do the presenters look like they have a good idea of what they are going to say rather than just fluff? Are the proposed presenters well-published people with something to say?
Hope this helps!