I just started reading this thread, so am behind and may (probably) will be repeating things.
How about a confirmation popup when someone tries to donate off the chosen path. That way the option remains for them to donate if they need to drop a badge just to get on that stage, but it might make them think twice otherwise.
This sounds good, but has problems and not just that it's another popup.
- Sometimes you want 2 paths done, but not the 3rd or you want all 3.
- Sometimes you want a path and the tricolors.
At times trying to run the FA feels like herding cats. (Though that can be done by dragging a fish tied to a string in front of them.) There have been members in my FS that are going to drop badges wherever they want. Do I want to kick them out? No. Also, it usually doesn't hurt and if it does, well, it's a game. Smile at the cat running off chasing a mouse. Maybe a gentle reminder of the gators off the path. It's a Fellowship Adventure. Joy of gaming within your FS is what's important.
Out of everything I've seen mentioned I like marking the path in a more obvious way the best. People pop in and out of the game and might not see the simple rings. Maybe make it so waypoints can be individually highlighted (Problems of mechanics including 2 people working on it at once). What I absolutely don't like is restricting badge placement.