Yes it was good of them to replace the Ditra, although there's no way I was going to shell out for a replacement when I was following the thinset manufacturer's directions... I would've switched to a competitor's product.
Schluter said to add water to the Kerabond until it's like yogurt--will hold ridges but they'll slump. When I said that Mapei says not to overwater it, they said it's not overwatering--it's just being at the top end of the water range...(!)
When laying the Ditra the first time, I pulled up the Ditra and it had full coverage stuck to the underside. The pattern on the thinset now on the floor confirms that I had full coverage (no trowel lines), but I suppose it wasn't wet enough to actually soak through the weave of the fleece.
I used the official Ditra 4.5mm square trowel.
Floor heat has never been on.
I used a float to embed the Ditra in the thinset. There is a very strong grid impression in the thinset, so mechanically I think I did everything according to Schluter's instructions. The problem seems to be that stock Kerabond won't wet Ditra's fleece enough for a strong (and quiet) bond.
I'll try the soaking methods for getting the old thinset up. My Novaplan Easy SLC is grey (covering the heating wires) whereas the Kerabond is white, so it should be easy to tell how far to scrape.
Interestingly, the hairline cracks in the SLC have *not* telegraphed through the thinset... Maybe I don't need an anti-fracture membrane after all ;-)