Ah, this discussion again. Always fun.
From my most recent read, I am currently of the opinion that Shizune's route "ends" with "The Summit". The next three scenes are epilogue - tying up the loose ends, reaching conclusions, etc.
Also, you're missing a key detail with even presenting the idea of "friends with benefits". First off, Shizune only has two friends. Secondly, a key tension throughout her route focuses on the fact that Shizune isn't good at dealing with people. She is so caught up in the moment at any given time that she has trouble thinking beyond the specific project she has in mind. Hisao
knows this, hence why he doesn't push for Shizune to make plans. Besides, Shizune is far less about
saying you'll do something, and more about doing it.
Also, something I literally just noticed. The last line of Shizune's route:
Shizune laughs, as if she doesn't have a care in the world, Misha's laughter joining with hers as easily as if it were her own. We'll meet again.
Shizune. Laughs. The girl who's stifled every vocalisation up to this point, to include her moans during sex. Laughs. In public. The girl afraid to have her voice heard, even by her lover. Laughs.
Five years of combing Shizune's route for the tiniest of details and insights, and I miss something so glaringly obvious. ><
It may seem trivial to you, but I'm
still trying to wrap my head around the implications of this.
If it's not clear, I'm in the camp that her ending is unambiguously happy.
As for Shizune's family, that's a bit fuzzier. All we have confirmation of is that Hideaki doesn't know sign language. Other than that, it's left ambiguous (with the admitted implication that they can't).
What we do know is that Shizune clearly has a working understanding of sound. She deliberately chooses to be nonvocal, stifling every laugh and even moan. Getting into theory and speculation, I'm in the camp of people that believe this indicates that Shizune lost her hearing in early childhood (no later than 6, given Jigoro's "twelve years" line). This raises the possibility that she knows how to speak (not well, as she doesn't have self-feedback), and, given Jigoro's comment of trying everything, she almost definitely knows how to read lips reasonably well (caveat: reading lips is far from exact, and many find it unpleasant and annoying).
Her possible faculties aside, Jigoro comments that Shizune refused to talk to him, but he assumes that is normal for children/teenagers (as opposed to "normal for deaf people"). Compounded with this, Shizune relies on translators, but only uses the phrase "tell him/her" in "Pangrammatic Window", when she wants Hisao to say something to Jigoro. Notably, Jigoro's immediate response is to ask whether it was Hisao or Shizune saying it, which heavily implies that he knows what Shizune said. In this context, it seems more akin to siblings in an argument with each other:
"Billy, tell Jimmy I still haven't forgiven him."
"Jimmy, Wally still hasn't-"
"I heard him, Billy. Tell him I don't care."
So there's a lot going on with Shizune's family, but I imagine next to none of it has to do with her deafness.