We could have a look at (an adapter for) VLCJ-Pro, to see what it would take to support users with a license.
Concerning your tests with different media frameworks: I cannot get the JDSPlayer to play .webm files. Disabling the Microsoft Media Foundation doesn’t make a difference: the JDSPlayer reports "Cannot play back the file. The format is not supported." in the log and the Player Info context menu of the video reveals it is actually a VLCJ player). DirectShow is a rather old media framework and I assume that without additional codecs installed, it doesn’t support webm.
With other media frameworks on Windows, I don’t see similar interactions with the system’s volume mixer (ELAN’s volume remains 100% there) or interference between different players (per player volume levels are maintained and applied). So, based on my tests I think that other behaviour is unique to VLCJ.
The problem of the video not showing up (instead a blue rectangle is shown) is not solved and the cause is not clear. Because there seems or seemed to be a connection with the aspect ratio not being recognized some additional logging was added. Because the creation of a JMMFPlayer is an asynchronous process, there will now almost always be one or two messages about the aspect ratio not being initialized, even if the player is created correctly in the end. And it can happen that the width and height are correctly read from the file (and all other functions return a "success" value), but the video still doesn’t show up.
On (my) Windows 11 I can now see something similar with the JDSPlayer (while this doesn’t seem to happen on (my) Windows 10, with the same version of ELAN).
Best, Han