As far as CDE, you COULD just do dressage, but you have to pay for and enter the entire thing, so you might as well do cones and marathon, too! That's the funnest part! :) It's a TON of fun :)
I use an easy entry cart with my minis in all phases of CDEs. I do believe your show cart would be fine for dressage, except that your wheels may not be. I know the club here allows wheels like that for training level for minis, and for our one day CDEs we can use them in prelim (it's not ADS), but really CDEs require non-wire spoked rims. You'd need wheels like this: http://farm1.static.flickr.com/58/18...104364.jpg?v=0
either wood, steel, ect.
As for his size, I have two friends who do CDE with minis under 30" and do very well at it! :)
Another website to check out is www.cde4vse.com
(and if you go there, the top right photo is my mini Chili Pepper and I in a water hazard :) )