I haven't used these commands, but I did notice an error in your batch file. Maybe that is just a typo for the post.
MC27.exe /Mode Mini
MC27.exe /Clear
MC27.exe /Play "\\NAS\My Music\\Beethoven\String Trios Op.9\String Trio No.1\01 - I. Adagio.dsf"
MC27.exe /Play "\\NAS\My Music\Beethoven\String Trios Op.9\String Trio No.1\02 - II. Adagio.dsf"
MC27.exe /Play "\\NAS\My Music\Beethoven\String Trios Op.9\String Trio No.1\03 - III. Scherzo. Allegro.dsf"
MC27.exe /Play "\\NAS\My Music\Beethoven\String Trios Op.9\String Trio No.1\04 - IV. Presto.dsf"
Matt has done some work on clearing Playing Now for Zone controlled by a Client instance. It is possible that those changes will fix your issues with local Zones as well.
Oh, why not. I have some of those changes, so a little test. I built a similar batch file:
MC27.exe /Mode Mini
MC27.exe /Clear
MC27.exe /Play "F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\01 - Dreamlover.flac"
MC27.exe /Play "F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\03 - Anytime You Need a Friend.flac"
MC27.exe /Play "F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\06 - Never Forget You.flac"
MC27.exe /Play "F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\10 - All I've Ever Wanted.flac"
Without "Options > General > Behaviour > Start playing when files added to Playing Now" set, and starting with content in Playing Now, and MC closed, all that happens is that MC is opened in Mini View and the four tracks are added to the end of Playing Now.
With "Options > General > Behaviour > Start playing when files added to Playing Now" set, playback starts at the first track in the batch file, even though previously the first track in Playing Now had been played and stopped, and so was still selected.
So the "MC27.exe /Mode Mini" is working.
The "MC27.exe /Clear" is not, but it may be getting applied to the wrong Zone. However, I was using the default Player Zone, so that is unlikely. More like it was being applied to no Zone. Also adding "|Zone=0" to the commands didn't fix the issue.
Also, the "MC27.exe /Play" command did add the files to Playing Now, but playback did not start. Also, adding "|Zone=0" to the commands resulting in the tracks not being added to Playing Now.
Changing the /Play command to a /Replace command resulted in only the last track being in Playing Now, and playback started, even though I had turned off "Start playing when files added to Playing Now". So something is working.
A batch file of:
MC27.exe /Mode Mini
MC27.exe /Clear
MC27.exe /Replace "F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\01 - Dreamlover.flac"|"F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\03 - Anytime You Need a Friend.flac"|"F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\06 - Never Forget You.flac"|"F:\JRiver MC AMINTHA Local Media\Music\Mariah Carey\Music Box\10 - All I've Ever Wanted.flac"
Added all four tracks to Playing Now and started playback of the first of the four tracks. If the Wiki is correct, that would indicate that MC thought the play state was Playing, not Stopped.
"MC27.exe /PlayReplace" didn't work though. Also, omitting the /Play commands, as per the Wiki, gives the same result as including it.
The Clear command wasn't doing anything though, even on its own as:
MC27.exe /Clear
or
MC27.exe /Clear|Zone=0
So I agree, /Play and /Clear aren't working. Zone selection also didn't work, and broke the <playitems> list when added to the four track list batch file above, adding only two tracks to Playing Now. The longer version, "/Command Play" also failed the same way as /Play.