Closing this report.  I investigated the remaining issue and it looks like a perfectly valid call to PyBool_FromLong:

/src/cpython3/Modules/_struct.c:509:28: runtime error: load of value 128, which is not a valid value for type '_Bool'

static PyObject *
nu_bool(_structmodulestate *state, const char *p, const formatdef *f)
{
    _Bool x;
    memcpy((char *)&x, p, sizeof x);
    return PyBool_FromLong(x != 0);
}