Update: I have a bit of good news and not so great news.

The good news is that I had some time to work on this again, specifically with isolating the failure in test__xxsubinterpreters.DestroyTests. Locally, I added a few temporary "@unittest.skip()" annotations to the other failing tests in an unmodified 3.8 until the only failure that would occur was the following:

======================================================================
FAIL: test_still_running (test.test__xxsubinterpreters.DestroyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/aeros/repos/aeros-cpython/Lib/test/test__xxsubinterpreters.py", line 764, in test_still_running
    interpreters.destroy(interp)
AssertionError: RuntimeError not raised

----------------------------------------------------------------------

Once the failure was isolated, I applied my changes on top of that branch with the skipped tests to ensure my proposed change resolved the above failure. Unfortunately, it still occurred.

It was somewhat disappointing, but I'm certainly glad that I waited to thoroughly test the proposed changes before opening a PR. I'll continue working on this and see if I can find another fix.