First, definitions, just to be sure:
In MC the Library is the set of index files storing information about the media files you own. It is associated with one MC installation.
The actual audio, video, etc. files are media files, and can be stored locally, on USB drives, on a NAS, or anywhere accessible to Windows (in this case) and hence MC.
The Library should
always be on a fast local hard disk, preferably an SSD, so that you get fast response from MC.
The Library should
never be placed on a NAS. Ever. There be dragons.
MC was doing the only correct thing it could do in your situation 1) above. If it didn't do that you would have corrupted the Library, which is basically a flat-file database and does not support multiple concurrent users.
JRiver's Client/Server model, and in fact any server model, requires that the server by up and running when you use the Client. Simple. Set it up to wake when the Client accesses it, using WOL. MC sends WOL Magic Packets automatically when a Client tries to connect to the Server using the JRiver Access Key method.
You can set which Library is loaded at MC startup. So no need to change Libraries. Just have the Client always open the Server Library, and wake the Server if necessary.
Have you ben trying to use the function under "File > Library > Sync Library"? Don't use that. It is intended to sync all media files and the Library between two PCs, so that they are set up identically. It isn't perfect. It is old functionality. I don't recommend it.
Use the Client/Server configuration. Set up Authentication with a Username and Password, which will enable the [Number Plays] field to be synced from the Client to the Server. Don't use Read-Only Authentication. Note that each time a Client connects to a Server, it receives a complete copy of the Server Library, including the latest information in the [Number Plays] field. Any changes made in that copy are then periodically synced to the Server Library, when using Authentication.
Read more Wiki and Forum threads.
https://wiki.jriver.com/index.php/Media_Networkhttps://wiki.jriver.com/index.php/Media_Server