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I'm not a concrete expert but have built many shower bases like your project (they are the best)- so I'll try to help.
Can't understand the concrete problem after seven days - should certainly be cured enough by now. Did you "play" with it too long? If so, the cement content of the mix may have come to the surface and can in fact be "dusted off". The base is probably ok but if you are doubtful about the finish (is it too "rough"?) you can use regular mortar mix to smooth it out. It has been proven that when applying a thin smoothing coat over existing concrete, a coating of "white glue" helps to bond the new mix to the old.
As for the sealer, I never use this, at least at this stage. I think that with your PVC membrane, concrete, ceramic tile and grout, the chance of water seepage is probably non-existant. However, as a belt and braces precaution, I usually give the finished base a good coat of silicon sealer (available in stores for this specific purpose).
Incidentally, although they are not as readilly available these days for some reason, I like to use 1"x1" mosiac tiles - they "mold" to the base contours very well.
Hope this helps and good luck. Alan