There's more to Bella and Jill's relation than meets the eye. The pictures you mention are indeed intended to make us suspect something like that. And that conversation between MC and Bella too:
And her reaction to that was...
We all know how Derek acts around a vagina, but would he really jump his own sister's girlfriend? I don't think so. Maya said she uses dildos, so it's only fair to assume that Josy does the same (and they probably use it when are with each other). And about her knowing "her way around a dick", well, her anatomy could be more suitable to take MC's dick than Maya. And Josy could also have had a boyfriend before; she never said anything regarding her sexual preferences.
About the cafeteria lady, she'll get hers eventually; she frequents DIK's parties, so I guess in due time we'll witness her getting some.