I think you answered your own question having them trying to switch grips from a western/Sw to a bh grip on volleys. From my opinion, that would produce interesting results.
I would teach them the continental grip since you dont really have time to develop their volleying skills. Have them go to the backboard and practice their volleys without changing grips. 15 minute practice. Teaching them one grip for all volleys would be much simpler for you to manage. You can work on footwork and meeting the ball in front of their body plane as they practice on the wall.
Since they are playing doubles, I would help them break the habit of serving with a grip other then the continental. See how it goes, if they are just not getting and not practicing it, then maybe go back.
Use your backboard to teach them how to serve with the Continental grip. 15 minutes serving with the new grip. Make sure you put targets on the wall for serving.
Former USPTA Teaching Professional
Volkl Tour 10 V-Engine Mid/Luxilon Big Banger