as you say, since Skiluros' empire stretched 'from the Thracians to the Maeotians' appr. 120 BCE, this was the one Khersonesos was up against,
****GK: Correct.****
thus Diophantos' campaign against them was just opening another theater of war in a war already begun.
****GK: In the sense that Skiluros' Scythians had already captured all of Chersones' holdings in the northwest Crimea (incl. Kerkinitis and Kalos Limen) and were assaulting the metropolis itself. Diophantes recovered all their territory as well as conquering the Scythian holdings in the Central Crimea. Palakus and co. [BTW an interesting tidbit: Strabo says that Posidonius held Skilur to have fathered 80 sons /shades of Attila.../ and Apollodorus says 50. The Neapolis inscription has... 60.] were pushed back north of the Perekop isthmus.*****
This was then the war which broke the Scythians,
****GK: Again, we know of no such expedition at that time. It certainly must have isolated the Scythians who remained in the Crimea from their brethren north of the isthmus.*****
resulting in the Tauric Scythians and the Dobrogea Scythians being isolated from each other.
****GK: We still have to figure out how that happened. And not just "isolation" but loss of political control by the Scythians in "Little Scythia" and settlement of the Bastarnae in Peuce... The emended Strabo + Plutarch does not suggest that the Scythians were involved as opponents in Mithridates' campaign of ca. 91-89 against Bastarnians and Sarmatians.****