It's a bit strange Yuu wanted to call Touko by her name in chapter 41 and indeed does it occasionally (when she isn't
saying just "senpai") but keeps calling her Nanami-senpai in her mind. Gotta wonder if Nakatani has a reason for this or just sees it as something perfectly normal so far.
Well, not wanting to fully dive into the complexity that is japanese psychology & sociology, I'd take it as Yuu putting Touko, her cool and beautiful girlfriend, on a pedestal, mentally, to the point she just can't think of her without honorifics and the regard that comes with Japanese using the other's last name.
I'm not sure if we can call this distinction "normal", but I'd consider it in-character and even relatable :)
...BV wasn't wrong. Responding to the pic of Yuu's imagination clinging to Touko's arm while fully dressed and the line
That is literally the furthest Yuu imagined anything.
with the actual, previous panel (besides it being discussed already) of Yuu picturing herself with an unbuttoned Nanami and touching her sensually, seems viable.