@jhughes I'm a little late to this thread. I am among those working hard at -28 and have just this summer gotten to the point where I can run it more than once in a 6-pass set. Please allow me to try to answer your questions based on my recent experience.
My answer to your first question is that I run -28 when I can overcome these three barriers: (1) Getting wide and way up on the boat at the gates; (2) Holding the handle in coming off the second wake; and (3) Countering by very deliberately tucking my outside elbow behind my upper body during my reach as I approach the buoy. When I remember these three things for the whole pass, I round five and come up on six with all kinds of time to spare.
To answer your second question, I think you have to be solid at -22 before this stuff makes sense. Getting wide at the pullout and countering during the reach -- among many other details -- are keys for running a solid -22, but they become absolutely critical on the yellow loop. But for me, it was squeezing both elbows into my vest as I come out of the whitewash that really and truly made this pass attainable -- FINALLY!! The lights just came on. Forgetting this at -22 isn't the end of the world, but you'll be scrapping to run -28 if you let the boat take the handle from you by not keeping your elbows in close.
One other thing about -28 for me is that this is the first time I've gotten the sensation of being "cast out" or flung toward the buoy. For me it seems this is where "real" slalom begins. You need to be up on that ski, outside elbow (or arm) tucked in behind you, giving the rope to the boat only when you're damn good and ready as you approach the next ball. You'll hear people talk about "light on the line" -- this, for me, is what that means.
Lastly, my answer to your third question is based on the fact that I ski in local grassroots/Class F tournaments. I want to be able to get 2 or 3 balls at -32 if I run my -28 in one of these events, so in training I'd shorten up to -32 just to be better acquainted. By keeping these same three concepts in mind -- wide pullout, elbows in, very countered -- I was able to run 3 at -32 in last month's Class F after a relaxed, clean -28. In training, when you return to -28 after a couple of attempts at -32, everything is calmer and seems to make more sense.
This is what's allowed me to start dialing up the intensity and finally get into -28 and -32. Your mileage may vary!