You might have used to much thin-set or the thin-set might have not been set just right.
RedGuard or any of the liquid membranes are not allowed with a Kerdi Installation. However - Ardex 8+9 can be used with Kerdi and Kerdi DS installations. Jim will most likely freak out over this advice but it is allowed. I recommend you do a couple things.
Try using a shop vac on the shower floor. If your kerdi lifts off you just saved yourself a looming failure. If it stays put. Pick up one box of Ardex 8+9 and a box of SK Mesh to re band all your seams. After calling Ardex for permission (you will get it) let me know.
I'll work on adding some pictures of this process.
If it causes you any distress, understand Ardex 8+9 is the equivelent to Kerdi Coll. Watch the European Kerdi installations you will see them using Kerdi Coll to seam Kerdi.
I have flood test over a dozen showers now with this system and love it.
I do not love so much how hard is the mix and have found that you need to add one part liquid to one part powder. Mix the powder into the liquid in thirds. 1/3 spin for 30 seconds. 1/3 thirty seconds more. final third then blend till smooth. Once smooth I mix three more minutes - let slake for ten and then re-spin for 1-2 minutes. This gives me the best mix with no dry patches.
SK mesh is perferated so it lays down nice and flat with no air bubbles. I shot a video today at work showing how I band Ardex 8+9 to Kerdi. I will upload that in the next day or two.
Ardex 8+9 is roughly $70 - $95 per box.
Sorry to hear about your troubles. Good on you for doing a flood test - this is exactly why you do them.
Did you set up an evaporation control test? This is the surest way to know if you where loosing water. JW