The last MC11.1 version worked with an MC12 license. I think it actually gets an MC12 license when you go on-line and try to buy it, but I have not personally tried that. If that does not work you should be able to buy the license with MC12 and use MC11.1 as a registered version on the same PC & OS.
A bought MC12 license should work with MC11.1.201 and all MC12 builds. I think that has not changed.
MC11.1 is a mature product that works extremely well. I still update my main MC library in MC11.1 and import that library to MC12 just to keep the same files and library data available in both versions. The last MC11.1 version can be installed simultaneously with MC12 without any problems. (At least without any problems within these programs. I have not tried third-party programs like Netremote.) The library files must be kept separate because the database format has changed in MC12.
The v. 11.1.201 is available from this link:
ftp://ftp.jriver.com/pub/downloads/music/old/MC111201.exeEdit
What exactly are your problems with MC12 and with Netremote? Have you asked help from Promixis?
Edit 2
If needed you can uninstall MC12 after it is registered. Its uninstaller does not remove bought JRiver licenses from the OS. MC11.1 will continue to find a valid registration after uninstalling MC12.