Yes you do go to rostov if you fail Kurasawa 2. Personally, I like the Rostov series since you get to fly Raptors (my favourite WC1 ship) with Iceman (my favourite WC1 wingman). You can still beat the game if you don't save the Ralari, but it will take a bit longer. You should still try to beat that mission though just for personal satisfaction. Just keep trying the tactics already mentioned and eventually luck will smile on you and you will do it.