Due to the somewhat rough texture of the mud pan, I recommend you use the V-notched trowel. It'll leave a little more thinset on the bed than the square-notch. If you'll have a seam in the floor, don't let that seam fall on the drain flange. After dry fitting your piece(s) of Kerdi, pull the Kerdi, then use the flat side of the trowel to burn a coating of thinset into the surface of the mud bed. The goal is to embed the thinset into the mud, while also leaving a veneer thickness of thinset behind. Then comb it off with the notched side of the trowel, with the trowel at a roughly 45-degree angle, leaving a nice combed surface. You'll get the hang of it within a few swipes of the trowel. Set your Kerdi over the thinset, then use the flat side of the trowel to embed the fleece into the thinset. Move the trowel parallel to the thinset ridges to minimize entrapped air bubbles under the membrane. As I embed the sheet, I work up to a somewhat firm pressure on a low angle trowel. You can start light, and move to more pressure. Again, you'll get the hang of it in no time. If you're moving thinset around under the Kerdi, which happens quite often, you can bring the excess thinset out from under the membrane at the edge of the sheet of Kerdi. For this photo I raised the edge of the drywall knife so you can see the thinset being pulled out: The goal is to get a smooth surface, no humps, no lumps, etc, with the material the same color. You'll see the Kerdi go from the bright orange to a muddied orange-brown as it embeds in the thinset. As you complete a section, I recommend you then just leave it overnight. For an order of work, it might be best to: 1) Protect the mud bed with cardboard 2) Kerdi the bench surfaces, wrapping/lapping the Kerdi on the bench surfaces onto the shower walls about 2-1/2 to 3" or so. 3) Then Kerdi the mud bed Let it sit overnight. 4) Protect the Kerdi on the floor with Cardboard, then 5) Kerdi the walls around the bench, and working your way out of the shower, 6) Kerdi the rest of the walls You know your shower best; the shape, access, the size of the area you have to work in. So you can shuffle that work order around as you see fit. The goal is to simply work your way from in the shower to out of the shower, and to not walk on any uncured thinset. If you feel you can do it all in one day, no worries, have at it. Just plan your thinset batches accordingly. You don't want your thinset firing off in a bucket while you are scratching your head trying to hang a sheet or detail an overlap. So it might be best to mix small batches, mixing a fresh batch of thinset for each task. If you do drop a tool and ding or puncture the Kerdi, just put a Kerdi patch over it. Seams need to overlap a minimum of 2", so...

It doesn't take much water to allow concrete to hydrate, in fact it can't be too dry and still be placed in the field. When concrete is poured the excess water rises to the surface as the aggregate drops. That water must be removed and the rest of the water in the mix is used up in the chemical reaction with the cement (hydration) which is often called "curing". That process was completed long ago so the concrete isn't giving up any of it's own moisture. It must be coming from somewhere else. The possible sources are earth and air. It's not hard to figure out if you consider each possibility separately. But first make sure that the downspouts are installed properly, all rainwater flows away from the house, and any foundation drains and sump pumps are working. It would help to know the climate, the kind of wood used (KD, GRN,) and how the air is conditioned in the house.

"It does sound like the moisture from the drywall mud floated up there and condensed on some cold roof sheathing. There is a huge amount of water in that stuff." If this is the moisture source (and leaks are not normally over a large area) then the problem will be 'fixed' as soon as you close everything up and the nmud finishes drying. Pre-mixed mud hardens by simply drying out (and shrinking as the water evaporates). Setting compound (mixed from powder) hardens by chemical reaction, and then the much smaller amount of water used to mix it up evaporates. If mixed with minimum water there is very little shrinkage (even in thick applications over an inch in thickness).

In my last home, on the advice of a landscape architect, we used pami pebbles for our aggregate walkway/patio and had the concrete mix tinted a medium brown-reddish color. It was a beautiful combination, especially in contrast with plant materials and a flagstone and brick accent wall. Got many compliments over the years. I didn't find it hard to walk on with bare feet because the pami pebbles are so smooth. We had our backyard patio installed a few years later by a different company and was talked out of using the same materials. Never liked the patio as much as the front yard - in looks or bare-foot comfort.