We have been trying to make sense of the first sentence of 7,4,3. What if we related it to what immediately precedes? I.e. 7,4,2.
We would get this:
"This harbour forms with another harbour called Ctenus Limen an isthmus forty stadia in width; and this is the isthmus that encloses the Little Chersonesus, which, as I was saying, is a part of the Great Chersonesus and has on it the city of Chersonesus, which bears the same name as the peninsula.
3 This city was at first self-governing, but when it was sacked by the barbarians it was forced to choose Mithridates Eupator as protector. He was then leading an army against the barbarians who lived beyond the isthmus as far as the Borysthenes and the Adrias; this, however, was preparatory to a campaign against the Romans.
So, then, in accordance with these hopes of his he gladly sent an army to Chersonesus, and at the same time carried on war against the Scythians, not only against Scilurus, but also the sons of Scilurus — Palacus and the rest "
What if the "isthmus" of 7,4,3 is not Perekop (as the editorial note suggests), but, as in the immediately preceding text, "the isthmus that encloses the Little Chersonesus which... has on it the city of Chersonesus, which bears the same name as the peninsula."? Everything would fall into place without the need to postulate an undocumented expedition at some point other than Chersonesos.