I can sympathize. Every year I take three neighbors out for a round. We always play later in the morning so as to not be in front of any serious golfers. I also tell them that the only rule is to play quickly. I tell them to improve their lies, move the ball to a spot they like and if that the ball looks mean at them it's a free pass to do whatever they want. I've also been known to kick their ball from the rough into the fairway. ;)
Even so, I'd be lying if I said it isn't a tough day just keeping track of where their shots end up and the like. But, I go into it just expecting that my guests will enjoy themselves, have a few laughs and (just) maybe have a shot or two that makes their day.
This I can manage, knowing it's only once per year.
As to your question about helping others with speed of play, it's a tough one with guys that play a couple times a year. Certainly you can set the example for others. Another way, is to complement the guy that moves the fastest so it becomes a positive comment that others will notice and (may) respond to. For example, one guy is quick to toss another ball down and whack it after the first one has gone 'bye-bye.' Just say something, like 'I love the way you play the game ... quick and quicker' If that all fails, sometimes I'll just say something like 'guys, we need to pick up the pace.' Then it's not directed at any one golfer.
Another thing I've done is used a scramble format. This way, everyone gets all their swings but they then play off the best ball. Hope that helps.