Sev will not know what to make of young Bella. Hmm, maybe she can find him in a few days? And maybe find signs that someone is there in the meantime? Yeah, just a touch but I think his spy instincts will kick in and he'll assess the situation.
Sev had enough of Peter that one summer he was forced to house him. :P
That's okay, Minerva, Sev does as well. I'd definitely be up for them having a decent working relationship since I generally headcanon that Minerva and Filius were the ones he got along with the best. (Explaining why they were the ones to step up and fight him in the end.) So having to do the things he did, knowing that he was going to shatter what he had with them hurt but... it wasn't as though he was given much choice. And he assures her that he never wanted to mbe Headmaster. He never wanted to be a teacher either, whether it was for potions or DADA. Again, not exactly his choice.
I think, once he unwinds his nerves and crawls down from the ceiling, he will be willing to offer some explanations to her. She may have to drag them out of him because he's more used to not telling people things than actually telling them but he wouldn't begrudge her knowing the truth about certain things. Though he does arch an eyebrow at her and remind her that after those wretched children broke into his office, he sent them for detention with Hagrid instead of the Carrows. He did what he could but his hands were largely tied.