I found a few references to it but nothing that seemed dramatic enough for me to read far into. Never even knew there was a such thing as upload mode until I found this. I was hoping maybe it was a splash screen for fastboot but apparently it is not. So far I have yet to find any hopeful functionality in this mode. But that doesn't mean there isn't any.Perhaps upload mode allows official service to update your phone, if their services are down and you need the update. That would be my guess. Couldn't find any explanation of upload mode online.
On this same device I have managed to make KingRóot the device owner using ADB shell dpm. Kingroot does have a receiver for device owner. However it doesn't seem to have an effect that I can see. Hopefully something else might have a better effect at escalating privileges I don't know. was hoping this would be enough to get some kind of privilege escalation to get root access. Beforehand I was also able to make icebox apps freezer device owner as well. The good thing about that is the application can be used to legitimately disable or freeze every application including system apps.