I recommend you post your question on the "Tile Forum". You'll get many more responses to your query, and faster too. Plenty of pros there who are willing to help. Good luck!

At this point you can do a flood test, but you won't get the same results as if you did it before. Now if there is a problem with the waterproofing it might not show up for months or longer.

I still did a flood test after tile & grout just to test how quickly the water would go down the drain/evaporate, not to test the waterproofing.

Check with your inspector--in my city, the flood test is performed as part of one of the many required inspections. If it is for yours, then you can feel more comfortable about it.

If Kerdi was used, you could. But you'd want to give the test more time than typical for it to be accurate.

During a normal test, when you plug the drain and flood the membrane, there are no porous materials in the shower pan, so the water level is what it is.It's going to stay in the membrane or leak through a flaw in the installation.

Flood testing an already tiled and grouted shower? You can flood it, but water will slowly percolate through the grout and saturate the grout and the thinset between the tile and the membrane. Some might even wick up the grout and thinset on the walls and the curb. So you'll have an initial water level, but the water level could drop a bit over the first day as water percolates between and under the tile. You might even see tiny air bubbles come up as air is displaced by water.

Once the percolating and wicking is done, the level should stabilize. With the exception of evaporation.