I use a similar key setup. Had the same problem initially.

To prevent slaves from randomly attacking whatever's closest when you don't have a target, use this as your condition:
[target=target, exists, harm] [target=targettarget, harm]

Using [target=target, exists] instead of [] as your fallback (for when they are leading) will keep them from always activating when target=targettarget is not true.

The only thing I could think of for the range issue is a range check. I have one of these on my Mages for sheeping. Basically I hit it, and it tells me in the chat window if they are within sheeping distance. This is useful since the slaves are a little behind the tank, and it can be hard to tell for sure without looking at their screens whether the ability is in range.

/script if (IsActionInRange(9, "targettarget")==1) then SendChatMessage("I can sheep","PARTY") else SendChatMessage("Can't sheep!!","PARTY"); end

If this is pull related, you could do the same thing on your tank, to make sure their pull is in range. In the above, 9 is the action id of the button where I put Polymorph. If this was to check range on your main, you'd want to change targettarget to just target.

Hope that helps.