In that case, using the Schluter valve membrane will help with potential water intrusion around the valve. But it still won't help with tile support behind the too-large hole, and as-is, a bit of the the Kerdi valve membrane will be sort of flapping in the breeze a bit as it floats over the gap. I'd dry fit the valve membrane over the valve and see how much unsupported Kerdi membrane there is. If the neoprene ring on the membrane fills a good portion of that gap, you could pack the remainder of the gap with stiff thinset, then pull more thinset out on the surface of the cement board, comb it out, then embed the valve membrane in the thinset, carefully smoothing everything as you go. Don't goop so much in the gap that it drips behind and gets on the copper plumbing or the brass valve body. The cured thinset will provide support to the previously "flapping kerdi", and the Kerdi will provide support for the thinset, tying it in to the cement board. So to speak. A little mutual reinforcement and synergy. So to speak. But again, if using larger tiles, it's not that huge of a deal. The edge of the tile could span that gap. Regardless, since you have it I'd use the valve membrane and thinset it to the cement board. Then when you Hydroban the walls, HB over the cement board and onto the fleece of the valve membrane. Just like the floor, where you'll HB down the mud slope and onto the fleece of the Kerdi Drain. Off-topic: Don't forget fasteners. Make sure you have adequate screws through the cement board and into the wall studs. Do realize that thinset has "thin" in its name. It's designed to be installed in "thin", not "thick" amounts. Reason being that it shrinks as it cures. The thicker the application the more prevalent the shrinkage. As to the Schluter membrane for the valve...they are primarily for protecting drywall cutouts when Kerdi is installed over drywll. But they can be used on cement board too. There's a neoprene ring in the center of the membrane. The thicker part of the ring (it sticks out about 1/2" from the surface of the Kerdi) goes IN the wall, the 1/2" thickness of neoprene essentially protects the 1/2" raw edge of the backer board from potential water intrusion. If the valve membrane fits your valve body, you can certainly use it, even though you have cement board.

Oh wow, thanks for all the posts!!! I forgot about my question on this forum...I'm on the appliance forum, and the kitchen one, and the flooring one. HA! Posting all over as I got lots of questions I need answered. Thank you for the pictures and encouragement. My walls in two bedrooms are not in good condition and I thought this would be an easy way to fix. I LOVE the look and it will be great for this 1952 lake cottage. Interesting though, I don't know that I have seen the bead board running in a horizontal fashion before. I like that, too! For those of you asking about bead board sheets versus individual boards, I posted a question about that earlier. There were many different posts that were interesting concerning pros and cons. Would be worth reading. Anyway, thanks to all.....and I would love to see even more pics of bead board. I heart it!!!

The fabricator was aware of our BS plans, which is with a thick stacked stone (which he assumed would cover the large gap). While our BS is thick, it's still not enough to cover, and what happens if I decide to go with a normal tile in 5 years? I also wanted to note the that gap is fairly consistent and caused by him cutting short, rather than wavy walls. The curious part is that he did a second template once he had the counters here, then trimmed those two edges in our driveway. I think the counters would have been perfect had he not trimmed them once on site. He goofed, but they are willing to fix it by sending out the drywaller. While waiting on the drywall guy, we started getting nervous about his plans on a fix. IMO, the best way to handle this would be to hang a second layer of drywall on both walls. We were worried that he'd come up with some sort of half-assed work around to avoid removing cabinets, so we went ahead and removed all upper cabinets to make it easier on him. He will be here tomorrow, I hope it all goes well as I'm really anxious to get this completed.

I have never heard of a 1/4 in. x 1/2in. x24 in. SS rod. Rods are round. That being said installing plate connectors, steel angles, wrapping in steel C-sections or even a carbon fiber wrap over the interface of the concrete/wood would add stiffness at the "joint". None of these suggestions are especially asthetically pleasing but all would be better than what's there. There is no other information provided about the deck configuation so we don't know the loads in the column. FWIW, bearing on the slab/concrete does not appear to be an issue as I would've expected the concrete to show some cracking or signs of distress already. Short answer to the OP. Constructing a concrete pillar like that and resting a post supporting a column is certainly not the correct way to build a deck support. It looks dangerous and should be addressed as soon as practicable. Call a reputable contractor in your area and get estimates on repair/replacing those supports.