Just to be sure, is \\server\music a location in the normal, local library on the client machine? (opposed to a shared library which WMP Tag Plus doesn't support)
Are you sure that WMP Tag Plus is loaded in WMP while the ALAC files get added to the library? If it is in fact loaded then this may be an issue in WMP Tag Plus. You can check by going to Tools - Plug-ins in WMP - the WMP Tag Plus item there should be checked (press Ctrl+M if you don't see the Tools menu).
Which duration does the library show for that particular song?Which codec did you install for FLAC playback in WMP?If you installed a DirectShow codec (which is most likely) then you indeed won't get gapless playback in WMP. WMP only supports gapless playback if the music format is being decoded by a Media Foundation codec. MFFLAC is such a Media Foundation codec for FLAC, so I suggest that you try that.
Yes, MFFLAC registers its own property handler, but one that only provides basic properties such as duration and bitrate, and none of the editable tags. Reinstalling the FLAC Property Handler should fix this.