Well, JRiver have been good enough to provide half the solution you asked for, with the addition of the /StartMin command line switch.
But I'm wondering if you should go back to your original statement of requirement and ask JRiver how they may do that. Or just look for alternatives, as you have selected one possible solution which may not be the only or best solution.
I'm looking to create a "clean" HTPC interface, i.e. you don't see MC at all.
You see, starting MC minimised, then keeping it minimised, has all sorts of window focus issues to manage, when applied to playing video. Audio is taken care of simply by starting only the server component of MC. Audio then plays, and no part of MC is displayed. Or at least MC can be set to run that way, by setting "Jump on play (audio):" to None.
I did a little testing, and video also plays without the User Interface running. You noted you could hear a video when you hit play, but no display is presented. I started to play a video from a client PC to my server HTPC with the User Interface closed on my HTPC. I could hear the video playing. I then pressed the "Media Center" button on my remote (same as the green button on an MCE remote), once to start the User Interface, and once more to switch to Display View. The video was playing.
So I'm thinking as an alternate solution to starting and keeping the MC User Interface minimised, if the server component could open a Display View window (full screen of course) when a video is played to it, and close that window again when the video is stopped, all your requirements would be met.
Of course this also may not be the best solution, or even possible. But it may be quite simple. That is up to JRiver to say.
I can understand why having a clean display when the screen is not in use would be good, if perhaps security video was being shown normally, or a slideshow effect not available in MC. So I think the idea does have some merit.
Also, it seems to me with a future increased focus on remote control of MC, from remotes and client devices, having the MC server service capable of launching any window without the User Interface being activated makes a lot of sense. In fact, if a developer wants to run an application without a User Interface, and to always run and be available in the background, it is almost always best to design that application as a Windows service, just as with MC Server.