I don't know what could cause the problems, but if you don't see the files on the NAS, try to create the whole database within a local folder first to see if everything works. If so, then the cifs mount is somehow related to the problem and we have to investigate further.
The APP_BASE_URL in the stack has to be the one used to access the system from the outside, so in your case just https://subdom.dom.ovh.
Currently not supported.
See this thread: Jopling Server 2.7.4 - Email Configuration
No, it is not supported.