Interesting, although I have a couple of issue with that speculation. First of all, Rika should know that if Rina tricks Rena's father, Rena would off her. She has seen that happen way too often and knows that it's effectively a dead end. Either she tried to prevent it and failed, or she didn't notice it. While I find the latter hard to believe, the normal signs (Rena going home straight after school) might've been different for some reason. As a result neither Rika nor the others would've noticed it.
Second of all, while Keichii was hiding something from Rena at the junkyard, that was while retrieving Kenta-kun and IIRC, murdering Rina would take place after it was successfully retrieved. So Rena has no reason to suspect that Keichii would know about it. Granted, seeing Keichii talk to Ooishi could still fill her with doubt.
What I find most difficult is figuring out what the motivation of Rena was to murder Keichii and then (expecting) to be "demoned away". It clearly seems to be to complete the curse, but would that still make sense if she had already killed Rina and/or Teppei? In Tsumi it seemed that Rena was rationalizing the murdering as atoning to Oyashiro-sama and expected to finally be able to live a happy live together with her dad. Assuming something similar transpired, what would lead her to continue murdering and even disregarding her own life?
That being said, none of the points really exclude the possibility and I don't really have a satisfying alternative present. From the start there was a weird focus on Rena if you ask me. As a result the "twist" in this episode wasn't really a surprise, but that might give us some hints into what actually transpired. In episode three, after the cotton drifting, Rena runs off "to look for the others", but moments later we see her watch Keichii and(?) Tomitake and Takano from behind the trees. So it's safe to presume that by this time something was already amiss. The directing leads to believe that she is mainly looking at Keichii. If true, that suggest the reason for killing (or at the very least doubting) him already took place. (Alternatively, since we don't see the remainder of the festival, she could've interacted with Tomitake and Takano and perhaps even ended up trespassing in the tool shed, but I highly doubt it).
My working theory was that Rena might've been able to remember things from other fragments, just like Keichii. At the junkyard the scene of Keichii hacking away to free Kenta-kun was clearly reminiscent of the scene where he beat Rena and Mion to death. During and shortly after Rena started acting weird, which might be after she "remembered" this from her perspective. This would be enough to be wary of Keichii (explaining other scenes where she spaces off, spying on Keichii during the festival, etc...). I'm not entirely sure how this would translate to her performing the curse. Perhaps she assumed that Keichii would be demoned away and she would be killed (not knowing Mion was also killed). Not wanting her dad to experience his daughter found brutally murdered, she decides to perform the curse herself, killing Keichii and being "demoned away" herself instead (which might be the lesser of two evils?). If anything, she was well prepared and given we saw these things in the kitchen before Keichii saw it, I don't think he was imaging this, unlike with the marker.
Obviously not convinced about the last part. But regardless of the actual reason, I think that is a red herring. It won't really bring us closer to the real question: who is the antagonist? It can't really be Takano as then Rika would know and (presumably) be able to stop it, but it would also ruin the original ending. The most logical motives would be either to torment Rika (or possible others of the main cast, although unlikely) or to get a hold of the Hinamizawa Syndrome. So while we, the viewers, are following the story of Rena and Keichii unfold, Rika is trying to figure out what is going on, Takano gets "replaced" and this change is ultimately too big for Rika to stop or prevent and she and Satoko end up being murdered. While they left the option of "suicide" open, that is a bit too predictable for Rika to do as soon as she would realize that this fragment doesn't lead to an happy end and would also not explain why Satoko died. (I'd also be concerned about how often Rika would be able to try new fragments, given how weak Hanyu has gotton, so even if it would already be a dead end, she could still continue to learn as much as possible)
Given the speculations of connecting this to Umineko, there are some candidates for antagonist. Perhaps this could even serve as background/origin for some...?
(Since this isn't a remake but an actual sequel of sorts, do we need spoiler tags?)