sadly with the arm time of freezing arrow, you dont have the luxury of getting a full duration scatter into a full duration trap. If you did i would have a GCD to do damage in between, but you dont.
I'm a bit confused here. You may have to waste a fraction of a GCD, but you can definitely time it so your trap "arms" right before scatter wears off.
Alternatively, you could scatter/trap the druid and wyvern the healer. On a side note, I'm not the expert on this so not 100% sure, but I'm somewhat sure that you should be using frost traps vs double melee teams.
This discussion has devolved into moot points anyway lol