Well, the important thing will be not not let anyone rush things... Rushing things gets horses hurt. Especially babies. You don't want to be in a tugging match with them now, where they flip over backwards or get cut on the trailer. Plan on it taking all day, and let them take their time, pushing them only enough to keep progress moving forward. Backing off if they panic. If Misty's in there, they may be naive enough to jump right in, or they may decide they don't like it and take a long time going. It just could go either way. Watch heat and ventilation if Misty ends up standing in there a long time, let her out. If her presence doesn't help them jump right in, take her out or take her seperately.
Quietex takes two hours to work.... Give it early and it lasts all day, so you won't have to hurry with it. It comes in a tube like wormer, or a powder you add to grain. Either is fine, the powder is cheaper. If you end up with the powder, I pour it in and then add just a bit of water so it isn't floury.
If they've never been tied, absolutely make sure the guy knows this and does not tie them. They'll go down and hang themselves on their halters, they probably aren't strong enough to break them yet.
Yes, if Mark is experienced, he'll know all of what I've told you, I don't think I've recommended anything different than anyone would do. They just might be issues you guys haven't faced yet so it's new.