> No. I do not think it is the real problem either. And I do not know
compiler behavior well enough. Actually, considering the setting is
still -O0 (aka no optimization) I am surprised it has any effect. if I
understood correctly "no return" is intended to help the optimizer make
"informed" decisions.

Does removing all no returns change anything for you? (It didn't change anything for me, if I did it correctly)
find ./ -name "*.c" -type f -exec perl -pi -e 's/_Py_NO_RETURN//g' '{}' \;