The plug-in manager does not scan at all. It reads entries from the registry.
The VST scanner (a separate program) builds the registry entries for VST plug-ins. The VST scanner can get messed up if given a bad scan path.
DX plug-ins are added to the registry usually during installation with regsvr32.exe.
A plug-ins can only be one format. The exception being PC and VST2 format. It is possible for one dll to show up in the PC and VST2 lists. This is because the PC format is based on the VST2 format.
Cakewalk does not install VST plug-ins in the "Shared" folders.