This one seems easier to deal with it. The ICC in Istanbul has 8 floors and the Istanbul proposal only uses 2 floors. So the ICC capacity itself is unlikely to be reached in DEVCon 7. Just keep selling tickets to those in the line paying.
Individual rooms in ICC will have capacity, so yes, not everyone can get in the main auditorium, but some of the rooms could be used for overflow. Overflow is likely only needed for opening and closing talks, since some people in the main room will leave to go to other talks.
I’m not an event organizer (but DEF CON shows what’s possible) so can brainstorm what you bring up. Assumptions like catering may need to be revisited. We may need to discuss some topics, like catering, and analyze from “first principles”. There may be vendor stalls in the venue chosen (like in a hotel or stadium), or there might be enough restaurants, fast food, (even street food?), hotels (those that have food) near the ICC. I will ask in the Istanbul thread.
In general, solving or reducing scarcity is very hard. Tradeoffs will be inevitable. We will also need to reduce assumptions to be able to explore and identify approaches.