"nor am i sure that it would actually have the privileges to unmount as itself."

if www-data can mount a drive, can umount this drive also (unless someone have mounted other partition over this)

"I want to unmount this drive as if i was trying to sudo as "other_username" and not "www-data"?"

Nonsense. This should never happen. What do you have in /etc/fstat?

it's mandatory to find out the password for "www-data"

Not, if you are root you can simply change it