What's the best way to continue if this happens again?
Pump your fist and/or do a happy dance. Seriously. Having the boulder aimed at Zorn's tank is the least stressful situation you can get, even better than having the boulder aimed at Toth's tank, IMO.
If you're concerned about losing aggro on Zorn (causing him to wander towards the Zorn DPS/healer, probably giving them Fearful when Toth jumps over), just fire your AOE taunt at Zorn while you're waiting for the circle to turn purple or during Baradium Heave's lengthy cast time. You'll still have your single taunt ready for Toth when he jumps.