No matter how and where you install Sonar, the plugin manager as a shared utility will be overwritten by the installer that comes last. And this will be the monthly updates for Platinum.
Assuming that you currently have a PluginManager installed that works for Sonar 8.5, do the following:
Go to the "Shared Utilities" folder for Sonar and rename "PluginManager.exe" to "PluginManager85.exe". This prevents that it is replaced when installing newer versions of Sonar.
The full path for the shared utilities is stored in the Windows registry under the key:
HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\Shared Components
If you want to be able to call plugin manager from Sonar, do the following:
1. In the Windows registry, export the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\Tools Menu\Plugin Manager
2. Rename that key from "Plugin Manager" to "Plugin Manager 85", and then change "PluginManager.exe" in "ExePath" to "PluginManager85.exe" and "MenuText" to "Cakewalk Plugin-&Manager 8.5" on the right side.
3. Import the registry file that you saved in step 1.
The tools menu is also shared, so you will now have two entries in Sonar 8.5 and Platinum. One will call the old plugin manager, the other the actual version.
I haven't tested this myself, because I don't have Sonar 8.5 installed. But this should do the trick, if the changed ID depends only on PluginManager.exe (and not VstScan.exe or some DLL).