sorry for the delay - been hectic here over the last few weeks.
mount method: via nemo > file > connect to server with the smb/windows share option. remember password is ticked.
terminal: unauthenticated fails (as expected). using credentials works (smb://user
if i run mpv via terminal, unauthenticated, then it simply complains about credentials used - which is what you'd expect. I dont know of a way to see what mpv is doing under double-click scenarios via nemo...
like i say, worked fine on FC25, but has never worked since FC26 fresh install. its possible something on the file server was updated at the same time, but client side was the only known change at the time.