At least to me it looks very obvious that he was stabbed and she's holding him on her lap
Yes, the implication seems to be that all three scenes are from Riri's POV of watching Ooma die.
Doesn't necessarily mean Riri was the one who stabbed him/her, but it does confirm that she was there when it happened, and who would better be able to sneak up behind him/her than Riri herself?
At the very least, I do believe that Riri is "on Ooma's side", or at least truly loves Ooma as she claims. That doesn't necessarily exonerate her from the first murder though...