What you really want is for the volume keys to change JRiver Internal Volume, agree?
Once you have achieved that;
Set the volume of your source application to 100% so that MC can manage the loudness and use the maximum source volume for all DSP, giving the best results.
Set the WDM Driver volume slider to anything you like except 0%, because at 0% it mutes all devices. I recommend 100%. Then leave it alone.
Configure the WDM Playback Device as you wish. i.e. Stereo out (or whatever your most common source will be) or 5.1, or 7.1 as MC will downmix to whatever your Xonar and other hardware supports as per your MC configuration, full range speakers, etc.
Set the properties of the WDM Driver as you wish, but under the Advanced tab, set Exclusive Mode on and give priority to exclusive mode applications.
You will already have set MC to Internal Volume, of course.
I assume that you will also already have set your Xonar as the Output Device in MC, and set it to WASAPI Exclusive Mode. Or ASIO I guess.
Now your volume can be controlled using the MC Volume slider. It can also be controlled using the source application (i.e. YouTube) volume slider, but that could interfere with the DSP, loudness control, etc. Best to just use the MC Slider, or keys that drive it, as per Stewart_pk's suggestion.
The WDM Driver is a little... fragile. For example, I disabled the Feature in MC, restarted MC as required, and my audio was still being processed by MC via the WDM Driver. That shouldn't happen. It could have been because I still had a YouTube page open that I had been playing from. I had to enable, disable, and enable again to continue testing.
Also, I found with the WDM Driver enabled and using System Volume in MC, none of the PC system volume sliders were linked to the MC volume slider. If the WDM Driver was not enabled and MC was using System Volume, the MC volume slider was linked to the Output Device slider. That is probably what made this process difficult. Of course you can't test that with the one available slider in the Windows tray, because that is the WDM Driver volume slider, and not an Output Device volume slider. I had to use the Volume Mixer on my PC to confirm that effect. See below.
Note also that when I left YouTube running in the background playing videos, sometimes, but not all the time, MC Volume Protection kicked in and set the Internal Volume back to 20%, to protect from overdriving the speakers. I don't know why that only happened sometimes, or at all, because it is only supposed to happen on starting MC I think, or maybe on new sources, which is probably why it has done it. Possibly a change of source audio format triggered it. I haven't investigated. happened a couple of times in twenty minutes.
Now, do you have a Volume Mixer installed in your Bootcamp environment, like the image below? If so, have a play with that and test what each of the volume sliders does. I suspect it will be enlightening.
In my image, left to right;
The left is the WDM Driver volume slider. It mutes all volume if set to zero, and drags the other sliders up and down, but doesn't change volume.
The next one is the System Sounds volume slider, and changes the volume only of system sounds, as expected.
The next is the Google Chrome volume slider. It does change the output volume but doesn't actually move the slider inside YouTube running inside Chrome. I suspect that is a Chrome Master Volume, which acts in conjunction with the YouTube volume slider.
The next one is the MC Application volume slider, but it doesn't do anything at all.
So as you can see there are a lot of different volume controls in the audio chain when you use the MC WDM Driver with other applications.