I don't know MT law, but I'd say being a minor, you can't OC to begin with. Most states (all?) you need to be 18 to own a handgun and POSSIBLY to even use a handgun without the approval and supervision of an adult.
To answer your questions, if you are stopped by a LEO, the best advice is to keep your laws regarding Open Carry in your pocket so you don't need to remember them. Some people keep copies they can give the LEOs who don't know anything about it. Most LEOs won't get you on the ground or anything, they'll just stop you and see what's going on.
I can't give advice on a proper holster for this.
If by 'the parents' you mean your own parents, I'd advise you to get them in touch with people who already open carry in your area, specifically because they know what dangers there are and how dangerous it is.
If you're carrying for self defense, I'd suggest 2 magazines at the MOST. One in the weapon, a second spare. Personally, i'd say a single mag. If you get to the second magazine, you're already in over your head.