So, is it possible to make question like this to be "objective answer"-safe, and how should it look like?
To make it "objective-answer" safe, you'd have to stop requiring so much speculation about what you need. Furthermore, by asking for a one-size-fits-all solution, you're venturing into the classic "best practice" bad assumptions.
You are asking for:
- an "instant network" stash that connects to the venue's router and handles wired networking for all the attendees
- topography and practical requirements for equipment features in such a network setup
- general enough to apply to most hackathons
- ...gather the primary, objective (i.e. based on sources, calculated estimations etc.) "to-dos" and "no-nos"...
However, you've tainted this for "objective" answers; see each section I've marked with
[subjective because]... Quoting from your question...
- reliable bandwidth is a must
[subjective because] How are we supposed to know what you mean by "reliable bandwidth"? How much is that? Between what endpoints? Is this only concerned with "internet" bandwidth? It would also help if you could explain situations when the bandwidth wasn't reliable enough. These days, even a basic pfSense gateway will give you at least 100Mbps in and out at large packet sizes (read the packet sizes used by ftp, scp, git, etc... do you really need more?
Furthermore, I'm struggling to understand how you'd have control over bandwidth provided by your venue unless you purchased it yourself. That's possible, but I can't imagine someone doing that for the small number of attendees quoted in the question. If you are purchasing bandwidth for each of these hackathons, that significantly changes the requirements (cost, setup / teardown, coordination with the ISP / venue's IT staff, etc...)
- low latency is a strong desirable (since there may be e.g. contests that are timing-sensitive)
[subjective because] Could you please quantify what you mean by low-latency? For what applications? To what endpoints? Again, real experiences when this has been a problem would be helpful.
At first blush, you seem to be making this much harder than necessary. Even a bunch of Wal-mart fast-ethernet switches will give what I consider low-latency within the venue. Outside the venue is of-course, out of your control unless you buy a circuit from a provider and/or start throttling users bandwidth. But, throttling users gets complicated quickly... and I don't get the sense that this should be a technologically sophisticated solution.
- the network might be spread out over multiple, close-by rooms
[subjective because] What do you mean by "close-by"? Can you guarantee that the rooms will be within 100 meters or 300 feet (including having to run things through the ceiling, if required)? FYI, 300 feet is the distance limit for most copper cabling you'd use.
- we're looking for a setup consisting of a central router (or possibly more than one?), some switches, and the necessary cabling.
[subjective because] Besides perhaps PAT/NAT, what exactly do you want this router to do? While we're on the subject of features, are we to assume you actually want to deal with spanning-tree, and knowing how much bandwidth each port is consuming? In other words, do you want a bunch of managed switches or unmanaged switches?
- what features of routers, switches and other possible component devices may help with reliability in environment like the one outlined?
[subjective because] You're basically asking for a poll for what features you need, but so far we have no way to know what you need. As I mentioned multiple times above, we have no idea about the practical problems you've faced at past hackathons. Are you worried about a few bandwidth hogs of your internet connection? Do you want utilization and error stats? Do you want to map users to specific switchports?
If you an address the concerns above, I think we'd be a lot closer to a question that gets "objective" answers.