So basically a fis-request returns a JSON response with basic statistics of every band found in the layer that you specified.
So in your case, the NDVI layer probably has 4 bands that is why you get C0/C1/C2/C3. C0 has statistics for band1, C1 has statistics for band2 …
I think it will be useful for you that your NDVI layer has only 1 band , and for that, you need to specify in the evalscript to output only 1 band which is the NDVI band. The evalscript in your configuration should be similar with what you have in the previous exampe
With that you should be getting NDVI statistics like these:
Yes, since sentinel 2 has a revisit time of 5 days see here so there are no images for certain dates
The evalscript that you used above does return 1 output band, but it is not the exact NDVI values that you need, it is just a visualization band . You can see this from your evaluatePixel() function. See the doumentation for the colormap visualizer here
To get the exact NDVI values that you need, just paste the evalscript below to your configuration