I may be wrong and most definitely am biased. But I understood the annexation of Spisz county to be considered a separate event that was later added/snowballed into/confirmed by the partition and thought to be something of quid pro quo for allowing the bar confederation operate from bases in the Habsburg realms (I think present day Slovakia).
Then the salt mines came later when the partition was agreed upon
Yes, as I keep saying, this annexation happened before an official agreement regarding the 1st Partition. Which makes argument about MT being reluctantly dragged into the partitioning idea rather hard to accept.
Not sure about its linkage to the Bar Confederation: it was acting against the regime so why would the regime compensate Austria for supporting the hostile actions against it?
As for the salt mines, Soloviev explicitly mentioned them as a part of the territory occupied in 1770. While he can be considered generally biased, he is considered quite reliable as far as the solid facts were involved.