> OOI, does it become a "blocker" again once you've done the alpha release, or what stops it being deferred past the beta or even the final release?

Check out the devguide:

https://devguide.python.org/triaging/#priority

> The issue will not hold up the next release, n. It will be promoted to a release blocker for the following release, n+1.

But in any case, I normally promote them to release blockers by hand and all of them become full blockers in the beta.