Usually you want to maximale a role rather than mixing different roles. Esp. in duos or groups it makes much more sense to have a specialist for each role rather than several jack-of-all-trades.
As for aiming catapults you should look for a target indicator on the ground. Most likely it simply wasn't in your field of view as by default catapults are set to minimum range I think. Aiming then is simply adjusting the catapult firing direction until the target indicator covers the location you want to hit. Keep in mind that the shots will not hit instantly upon firing, you have to preaim quite a bit if your target is moving.
Used to play: 85 Champ / Captain / Runekeeper / Guardian, Guild Master of everything but cooking.
Playing now: Hellcat / King Tiger / GW Panther / IS / KV-5 / M4 Sherman and more