I heard changing targets was hard. Start AoE on a mob, then swap to a melee, or start it on a tank if they are tanked. If the mobs fall over within first 5 seconds of first mind sear channel making it break and wasting your mana then your support on AoE is clearly not needed and you can focus on the task you are far better at, boss pewpew.
9 out of 10 cases only the aggro target of the nasty-effect-mob gets hit by the nasty effect. 5man faceroll fight is faceroll. In other two examples you gave there is no reason for melee to not help on the AoE if it's needed.
On the other hand, if the mobs are moved out of initial AoE area your MInd Sear follows them. Everyhting has pluses and minuses, if the mobs run out of blizzard-like AoE the caster has to break it as well. Also, if the tank is the one moving out you can assume the mobs are probably going to follow.
Really, the only issue with Mind Sear is it's damage. Ever since it's targetable on friendlies "my target died" is MUCH less of an issue, and having your AoE move with the mobs is hardly a bad thing.