When did the extraterrestrials salt the oceans, was it prior to the first annihilation of humans on the earth or after?

It was indeed, as you have seen intuitively, a factor in the demise of human and was part of a broad assault to truly exterminate them as a species. This was done as a kind of parting gesture to squeeze humanity down, insult them in every way, create despair, and dishearten them. This is often necessary in what they do, to create a softening up, a weakening of resolve, a resignation to the inevitability of a dark fate. This always helps them be successful in such extreme subjugation and suppression. And they usually do this via external means as they do not risk direct confrontation because this creates personal liability for them and their safety. And as they are cowards, they must do things indirectly, to maintain that safety margin of bring once-removed.