Ben:
I've been thinking about my answer to your question #3 for a couple of days, because it's a good question and it deserves a good answer, and I really didn't have one to give you.
After further deliberation, I think the correct answer to your question might be that there really AREN'T any courses where a relatively small number of bunkers, by themselves, have a great effect.
Most great courses seem to have more bunkers than average -- as a non-golfer might expect, if you told him that a great course is supposed to be especially challenging. To be sure, there are some great courses which have less than an average number of bunkers. I'll run through a few of them to illustrate my point:
1. Augusta National had only 22 bunkers when it opened. But when you've got Rae's Creek and 100 feet of elevation change and greens like that, who needs bunkers? The bunkers that ARE there do not have the "profound effect" you asked for.
2. Ballybunion (Old) has not many more bunkers than Augusta. But they've got the wind, they've got the course falling into the ocean on four holes; they've got the biggest dunes in golf; and they've got small domed greens with shaved banks to both sides. Putting bunkers at the sides of the greens would've made the course EASIER. And again, the bunkers that are there are not very compelling ... indeed the best holes are the only without any bunkers.
3. Commonweath Club, in Melbourne, has fewer bunkers than most of the Sand Belt clubs, because MacKenzie wasn't there and they were working on a tighter budget, I imagine. It might be the best answer to your question that I could think of ... the bunkers ARE compelling, but there are probably 60 or 70 of them, so that's not such a small number. However, the bunkers by themselves would not be nearly as compelling if the greens weren't tiny and the bunkering wasn't backed up by trees (too many trees, but trees).
4. Riviera -- same as Commonwealth. The bunkers are compelling, but much more so because of the sycamores and the greens which compound their effect.
5. Royal Ashdown Forest has no bunkers at all, but when you have great natural roughs like that, who needs 'em?
I had a few more examples -- Royal County Down, the Valley Course at Royal Portrush, Rye, Liphook, the Eden Course at St. Andrews, Royal Worlington & Newmarket, Woking. All of them have a paucity of bunkers, but at all of them, it's really the undulations of the fairways or the greens that make the course compelling, and the bunkers add gently to it, compounded by great natural roughs.
In the end, it would probably be better to think of this question one HOLE at a time, instead of one course at a time. If you look at it that way, you'll see that the holes which have other natural hazards have less need for bunkers ... a single fairway bunker might be plenty if there is an important tree or some good native rough or undulations defending the other side, or if you are playing toward the Road green at St. Andrews. But when you get to the duller stretches of the property, you will not find many great holes that don't have three or four bunkers to enforce some strategy.
The problem is only that too many architects think their whole property is dull, and many have given up on adding trees or mounds or rough or any type of hazard other than bunkering. At Old Macdonald one of the best things we did was to add some undulation for strategic purposes on a couple of holes, instead of more bunkers ... on any other site you'd have to call it mounding, but in Bandon we could make it look like natural dunes, so we got away with it.
I'll hold off on your fourth question for a while as I am sure this post will provoke a bit of response.