I was under the impression that Sertorius was in communication with Sparticus and with Mithridates in Pontus. While I know that Sulla returned from Spain just for the leftovers of the Spartican revolt It does illustrate the kind of legion shuffling that they were doing to deal with different crises. Sertorius was only about fifty when he died he might of had ten god years in him or he could of died like Alexander. As long as he lived he was a sore on Sullan Rome. If he had ten more years he could have done a lot. especially if he had found out and killed the conspirators.
Basically I see the situation like how Hitler lost the Battle of Britain. It was the opening of the second front that doomed the Third Reich. He prevented them from focusing there full attention on Sparticus and Mithridates was ready to renew hostilities too. Im just not sure If Sulla could of maintained a two front war. Remember that Sertorius was a popularis who objected against some of Marius's tactics and was popular. One of the conspirators of his death offered, according to Plutarch, a list of patricians in Rome ready to rise up against Sulla. Even though the list was burned it showed there was discontent in the eternal city. so Sulla was possibly facing invasion from Spain (another Hannibal parallel) invasion of Asia Minor and Greece by Mithridates, and also a post-Spartacan uprising within Italy itself from Popularists.
-whew-
And yeah ur right Sertorius would have never been able (or willing) to detach Spain from Rome
His aim was to reconquer Rome from Sulla.