The gluino plays a similar naturalness role as the stop, at 2-loop level.

Do you have a paper to back that up? It seems very surprising that a 2-loop level effect would have the same constraints as something at the tree level.

Just beyond the limits is not an arbitrary choice. Given our current limits already, naturalness points to these masses being as low as possible.

Not quite. Given our current understanding it would appear more natural to have SUSY at a lower mass scale but if we find SUSY at 10TeV all that means is that SUSY is perhaps less natural than it could have been. It's like tossing a coin: how many heads in a row do you need to get before you conclude that the coin is weighted? You can draw an arbitrary line in the sand and say '5 sigma' but it is just that an arbitrary line in the sand. With SUSY we have the same problem: you can put an arbitrary line on the energy scale and say "above X TeV it is unnatural" but it is just that: an arbitrary line. You might be happier if SUSY existed at a lower energy scale (I would be too!) but the universe is not there just to make us happy.