From what I understand from the files you provided, the SES units does not have any disc drive on the sata port.
Instead, the port is connected to the Linux Box that act as a 'Virtual Drive'.
Probably similar as the console with built in HDD to emulate the disc drive.
The PC is used to connect to Nintendo server using openVPN to download promotional videos, demo, firmware updates.
When downloaded, the files are extracted, decrypted, moved to a specific folder and an task entry is added on the local mysql server.
A cron job is run at a specific interval and when a new task is found, it 'probably' mount the disc image on the virtual drive.
When the *.wumad is loaded on the Virtual Drive, the WiiU install the game demo or the video on it's NAND.
When you extract the wumad image, you see that the only function of the package is to install the content located on the META folder on a nand.
So let say you were able to burn the *.wumad image on disc and run it on a console, it would automatically install the 'demo' on your nand, you would not play the disc itself, it is just an installer for content inside the disc.
When completed, the entry on the mysql server is updated to completed.
The Linux box download an installer that install the content on the console memory.
Also, the USB Key is used for the certificates/keys used for unlocking the hdd(?) and to manually update the unit.
The box is expecting a USB key with the 'label/name' of SESWURIDUPD with a (VFAT?) partition. If it find specific file format on a specific folder, it will copy the content to the pc so it is processed, so the WiiU install the content.
Nintendo also had the ability to set an expiration date for a demo.
If the expiration date as been reached and the demo is still active, based on the mysqld entry for the demo, a command is issued to the SES Unit to uninstall the title.
If no expiration date is set, the computer will request an uninstall on or after 2036-12-31 00:00:01
Regarding the missing demo, if you have CLI access to the Linux box, it is feasible to have the video and demo reinstalled on your console and also change the expiration date up to 9999-xx-xx.
The unit uses development keys so it is possible to install additional things on the console.
Communication between the unit and the PC is done by /SES/bin/ses_driver
Again, this is what I understand from looking quickly at the files. It may not be 100% exact but I will update when find additional information.
What is the USB error you get on the console ?