A GetCapabilities request must include the service type, so https://myserver.com/geoserver/mynamespace/wfs?request=GetCapabilities is not correct, it should be:
This will give an XML result in your browser.
Similarly, a DescribeFeatureType request must include the ...
There are a couple of LocationTech Projects which adress this, I think the most mature one is GeoMesa, but also GeoWave is able to store raster data in an distributed key/value (accumolo is mentioned most often here) store and connect it to geoserver.
preferring the Big Spatial Store Option: I think you should have a look at Geomesa Raster tutorial
You don't want to cascade a remote WFS (that is republish someone else's layers in your own GeoServer). You want to use the GeoServer REST API to add and publish a shapefile.
So you need to look at this tutorial from GeoSolutions
It's because you are not making a valid WMS request - try something like
or if you want ...