I just tracked the buffer size not providing choices thing down in my own windows 10 laptop with a Realtek built-in audio driver. This is apparently due to the stupid Realtek driver… if you go to Device Manager, under Sound,video,game controller->Realtek Audio, right click on that, then do Update Driver, then do “Browse my computer for driver software”, then choose “Let me pick from available drivers”, then choose the option the ISN’T the Realtek driver, it will probably be High Definition Audio driver from microsoft. Then reboot. Then the Windows Audio Low Latency choice in the JUCE device manager will actually let you pick different buffer sizes, and it actually seems to work down to 128 (and have noticeably lower latency than before, so it actually does something).
This is one of those cases that using the generic microsoft driver is actually better than using a vendor’s driver apparently. You might want to pass that on to your user… it could help them.