Will this continue to be enough space - i.e., is the Dev size going to
be enough?

 +2042  #ifdef MS_WINDOWS
 +2043      PyStructSequence_SET_ITEM(v, 2,
PyLong_FromUnsignedLong(st->st_dev));
 +2044  #else
 +2045      PyStructSequence_SET_ITEM(v, 2, _PyLong_FromDev(st->st_dev));
 +2046  #endif

 +711  #define _PyLong_FromDev PyLong_FromLongLong

It seems so - however, Is there something such as PyUnsignedLong and is
that large enough for a "long long"? and if it exists, would that make
the value positive (for the first test).

Surely you can answer this second question yourself?  You do have full source to the CPython interpreter.

To answer your question: there is no PyUnsignedLong.  Python 2 has a "native int" PyIntObject, which is for most integers you see in a program, and a "long" PyLongObject which is those integers that end in L.  Python 3 only has the PyLongObject, which is used for all integers.

The PyLongObject is an "arbitrary-precision integer":

https://en.wikipedia.org/wiki/Arbitrary-precision_arithmetic

and can internally expand as needed to accommodate any size integer, assuming you have enough heap space.  Any "long long", unsigned or not, on any extant AIX platform, would be no problem to represent in a PyLongObject.  You should use PyLong_FromLongLong or PyLong_FromUnsignedLongLong to create your PyLong object and populate the st_dev field of the os.stat() structsequence.

/arry