For the record, in neither of the two quotes presented in this thread are Malachor V or Korriban (or any location with a given significance of darkside presence for that matter) stated to be requirements for Sion's ability to work.
"As long as the dark places of this world flow through the cracks of my flesh. I can not be killed."
Malachor V's darkside energies can only neccessarily be said to be required for Darth Sion's power to remain active to the extent that, at that particular point in time, the only energies he had access to that he needed were those located on Malachor V. It's entirely possible that the reference to Malachor V was incidental to the basic ability of preventing himself from dying, and determinent solely to the ability to prevent himself from dying as it related to his given situation. For Sion's power to remain in effect, energies are required, and as he is located at Malachor V, the "dark places of this world" are specifically required (the dark places either because Sion requires darkside energies specifically or the lightside presence of the world wasn't adequate). The requirement could be entirely situational. The extent to which Malachor V's darkside energies are required are not specified.
"There will be another time. But it is not now, not here, while Korriban runs through him."
All that is neccessarily being said here is that Sion would not be defeated there and then, but at a later date. It is not neccessarily being said that Sion would not be defeated there and then because they were located at Korriban, but simply that it wouldn't happen there and then, where they happened to be located at Korriban, but at another time. It's again entirely possible that the reference to Korriban was incidental to them not being able to defeat him there and then, and not determinent to it.