I chalked it up to time constraints being the reason why Hitomi and Kamijou weren't seen much. Outside of Madoka and Homura, everyone else probably saw Sayaka's death as a tragic kidnapping/murder, so I doubt Hitomi would have ever considered she might have had something to do with it. In the final timeline, I was under the impression that Sayaka died/vanished/was saved by Godoka at the same time Kamijou was playing and Hitomi was watching from backstage, so they probably wouldn't have found out about her disappearance until afterwards. Also, apparently in the upcoming PSP game it's revealed that Kamijou doesn't have any feelings for Sayaka at all, and even outright rejects her. :(

As for why Sayaka wouldn't purify her soul gem, well, you're right: it's because she's an idiot. She felt like using the grief seeds would be admitting that she was wrong and Kyouko was right, and she's still stubbornly clinging to the idea that she can be a selfless warrior of justice and didn't need anyone's help up until the bitter end. And Kyuubey is a bastard wanted her to turn into a witch, it helps him get his energy quota plus it served his ultimate goal of snaring Madoka and getting her to turn into a witch, so of course he's not going to help her beyond the bare minimum. bastaaaard.