Wait for Russia to wipe them completely out before deciding whether to liberate. At that point if you have the units and they are doing nothing other than sitting around (pplus you think you could win the fight), it could be worth it. On the other hand it may just make turn times take really long.
Russia would be unlikely to declare war on the liberated civ a second time, in my experience. The biggest reason to revive and liberate in this situation, IMO, is to friend the revived civ, forward cash to them, and get an extra Research Agreement out of the deal.
BTW a really underhanded thing to do would be to take two cities, but only give the Ottomans ONE city back. Hand the other one to a fourth civ you want to generate tension with Russia, who has little chance of defending it. Now Russia has to pass through that territory to get back to the Ottomans, which at a minimum requires Open Borders, but could also result in the Russian alliance dissolving into chaos if Russia decides to invade.