You don't need (or want to) deselect Exclusive Mode. As Jim points out, and I'm sure you understand, you are using the WDM Driver to play external sources via MC to your Creative SBZ Series sound card in Exclusive Mode using its ASIO driver. I assume that card is always in Exclusive Mode if you use the ASIO driver. All of this is good.
You were on the correct track with this:
This has happened before and usually some combination of restarts, Selecting and Deselecting WDM, and appointing JRiver as default sound source in windows fixes it, but I have tried all to no avail. I only get sound from JRiver MC.
Sometimes getting the WDM Driver to install and work again after a MC Upgrade, and less often an Update, can take a bit of fiddling. Nobody has ever been able to explain why. I think it is a Windows thing, not want to set the driver.
There have been a few posts and methods to get it working. A search might find the posts with a workaround that works. Usually, your method works.
Try selecting something other than the WDM Driver as the Windows Default Device, then switching off the WDM Driver in MC Features, restarting MC (maybe the PC, but shouldn't be required), waiting just a little bit (say a minute), then restart MC, switch on the WDM Driver in MC Features, restart MC, set the WDM Driver back as the Windows Default Device, and see if that works.
If not, after switching off the WDM Driver in MC Features, reinstall MC26.0.20 over the top of the existing installation, and then try switching on the WDM Driver in MC Features again, restart MC, set the WDM Driver back as the Windows Default Device, and see if that works.
Also, have you stopped MC25 from starting with Windows? The Upgrade to MC26 should have turned that off, but it doesn't hurt to check. If you haven't uninstalled MC25, then start it now and turn off the WDM Driver in MC25 Features, then try the above in MC26.
Otherwise, have you configured your security software to allow MC26 to work after upgrading to MC26.0.19? If not, your security software could be blocking MC from properly installing or running the WDM Driver, even if it appears to be installed correctly.