I just did a shower very much like yours with Kerdi for a local prep school here a few weeks ago. There's no need to fir out the walls at all. Just kerdi right over the block. I'm showing this last pic only because it shows the Kerdi drain. I wanted to take progress pics of this as I went, because I KNEW this would come up, but I forgot to, so there's my proof it's a Kerdi shower!! :-)

One step that is often ignored in stucco is letting it cure until the surface is no longer alkaline. It takes 60-90 days, and any paint applied before then usually falls off. Pick a paint line that is meant for stucco - one that specifies stucco on the can - and follow the prep directions on the cans. And apply it with the right kind of roller and not too thin. For longer-lasting color, start with a slightly grayed color, because the brights and clear colors fade to grayish anyway.

The use of sealants on brick is a contentious issue in the industry. I should note that the Brick Industry Association advises that sealants are the last resort. See Technical Note 23A (Scroll to Search function on left.) In the UK, the Brick Development Association recommends against any sealant period. If you do choose a sealant, use a penetrant silane or siloxane not a film type, usually silicone. Silicone sealants stop the water from getting in, but they trap water that is in the brick and coming through the brick from inside the structure, resulting in spalling. Brick spalling--the popping off of the outer layer of brick--is often caused by excess water trapped in the brick that freezes in cold weather. Source: Carepro.ca After personal experience destroying heritage bricks with silicone sealants in the early '70s, I avoid all sealants and only gently dry brush efflorescence. Rather than treating a whole wall, work with a small section, preferably not front and centre, and see what happens over a year. You don't want to attack a cosmetic concern and end up replacing masonry walls.

My SIL and I took out enough bricks and concrete blocks to make a doorway. Just bash with a sledge in between the webs and they crumple pretty quick. Had no problem with the rest of the wall. I think if you want a smooth end on the wall you have to replace all the blocks on the end, not just the half blocks. Then you can get all end blocks and half blocks for a smooth end.