Hi! Your post about your colt being mouthy reminds me so much of my colt Zane. Except Zane is only 2 months old. I hope I don't have another 1 1/2 years of mouthy to look forward to, lol!
He does the exact same things, tries to nibble on my back when I pick up his feet, chews the lead rope, chews my clothes, chews all the other horses tails. Sigh!
I correct him in a similar fashion too. Either flicking his nose or bumping his nose with my hand when he tries to bite. Chewing I try to divert him away from whatever he is trying to chew. Today I was ponying him off another horse and he was trying to chew my saddle blanket, and when I got home I saw he had chewed my leather saddle strings when I wasn't looking. He isn't being mean either, he has only used his teeth on a few occasions, but I don't want him to really start biting, know what I mean? So I am trying to nip it in the bud early. It's like he knows it's wrong but can't help himself!
I taught my baby to lead with a butt rope, which is just a rope that goes around their rump that you can pull on to encourage them to lead if they aren't leading to halter pressure. But at one and a half years, I don't know if that would work. Hopefully someone with more experience can tell you that, but I have a feeling he is a little old for the butt rope technique. I am raising my first colt, however I have a good friend that has raised a bunch of them and she always uses a dressage whip to help teach them to lead. If they lag or refuse to move, they get gentle tapping until they do move. I have my colt leading without a butt rope and with a dressage whip already.
I think your idea of the tarps, balls, etc. is a good one. I did the same thing with my baby when he was just weeks old, and he thinks tarps are toys! He also has a couple of Jolly Balls that a neighbor lent me, but me bites them more than anything else. He doesn't toss them around like I hoped he would. But like I said, he loves the tarp and anything like that (even feed sacks).
It sounds like you have a really good game plan. If it were me, I would start with the leading, because that is the foundation for all of his other training. Sorry I can't help you with much detail, because this is my first colt training experience, and my guy is much younger than yours. But here are some tarp photos for you to enjoy!