is it possible to argue an objective, i.e. undeniable, point in a non-objective system?
I can't imagine how. This is why governments are finding it harder to
stay in power without resort to force. They want a "flexible" system of ethics
for their own policies(pragmatism), but insist that ordinary citizens must
respect the law as if it were sacred.

University professors want to teach that there is no real definition of right
and wrong, but would be indignant if one of those students put that
philosophy into practice and robbed the professor.

in the end it all comes down to these three:

- God
- Yourself
- Others

Make your choice.
Exactly. And it just may be that one of those systems is metaphysically true
while the others are truly wrong and immoral. So long as there exists the
possibility of an objective system of ethics, there is always the possibility
that my actions may come under the scrutiny of someone more powerful than I am.
And if that person happens to own that system, then, in spite of all my
protests, I am SOL.