YT stream key - best practice?
Is there a way I could have pushed my content through to the original URL (barring the big collapse)?

That's not possible unfortunately, once you stop the live event the content that was ingested is made available for on demand streaming and no more live content can be streamed to YouTube.
If you think about it, it would be difficult to do otherwise, because multiple sequences of the stream would need to be stitched together and that could be confusing for users who watch the recording.
The thing that YouTube tolerates by default is the fact that you might stop and resume ingesting content after you press the Go Live button but before stopping the event. I say &quot;by default&quot; because on Facebook, for example, the default behaviour is actually the opposite: if you accidentally stop streaming in OBS or have a network issue, the stream is stopped automatically and you cannot resume it. (I've seen this happen a number of times, it's a quite common problem.)

The system I was part of (here's a stream key, you only have one shot at it) seemed sort of fragile and I wondered whether I'm misunderstanding my role in it.

The stream key is simply a string that you use to authenticate with YouTube, and YouTube actually allows you to reuse the same key between live streams!
What you should do in your case is simply make sure that the Go Live button is pressed at the right time :)
If it's pressed too early, there's no point in stopping the stream! Going live 20 minutes before the event is perfectly fine and it's actually a very common and recommended practice. While users wait, you can show a placeholder message like &quot;We'll be live at 7 PM&quot;. This allows you to check that everything is fine in practice with actual YouTube apps (cough November 11 cough), and also gives users a sense that everything's fine and the event is going to start.