If you haven't already put it up, just put regular drywall on the walls...you could use 5/8" stuff if you wish for a little more stiffness, but believe me, the 1/2" drywall is plenty stiff after the Kerdi and tile is on it. The cbu works, but it costs more, you'll have more seams, is heavier to move home, and harder to cut. CBU will work fine, though. Make sure to either spritz it or wipe with a sponge or it will absorb too much water and give you grief when installing the Kerdi. If you choose drywall, clean it off with a damp sponge to remove any dust before applying thinset. Don't let the thinset skin over before you attach the Kerdi - same is true when installing tile.
When mixing the thinset, make sure to use a premium dryset thinset. Also, mix the thinset with the larger amount of water they specify on the bag and mix it for the time they specify. One thing we learned was that the thinset literally appears a totally different product if it is just mixed verses mixing it the time the manufacturer specifies.
Protocol says, one project, one thread. It is really hard to keep the subtle history straight if you have multiple threads on one project. If you don't get an answer, respond to your own post and it pops back towards the top.
Don't try to use pieces wrapping around corners until you get the feel for it. If you don't have one, get a drywall taping knife - knock the sharp corners off with a file, grinding stone, or scraping it on a concrete floor; this helps to prevent catching them in the Kerdi. Doesn't need much, just don't want it sharp.
Make sure you get good coverage, fully embed the stuff, and pull back a section to check for 100% coverage. You can live with less, but there is no reason to not get it. You can't live with less than 100% on a seam. There is no required order of installation, you can overlap the seams any order or direction you want as long as they have at least 2". Strange, but the stuff is hydroscopic...it repels water, and with a tight seal on the thinset at a seam, water will never penetrate more than 1/4" (and then only with standing water over a long time) - so you have an 8x safety factor.