Not that dead don't worry I'm here :) Did you not write "Cinder fails" on purpose ? I bet the CRWBY would have used this pun in RWBY Chibi if it was correct haha.
Back to your topic and your options.
Option 1 - Cinder is indeed dead : In that case, rant would indeed be very well possible and even justified I would say. Because we would learn her death indirectly (CRWBY, maybe Salem somehow) which would be weak. If a character dies, his/her death has to be made clear in the show.And so far, the CRWBY has always made sure to make a character's death clear ; I don't see them breaking this rule. It would also be weak because this would be very unsatisfying regarding Cinder's character ! No real backstories, no clear motivations... This would damage her character. I don't remember when we hear this but at some point we hear Cinder offscreen saying something like "I want to be powerful" to Salem I guess. This is a motivation but the poorest possible without any other justifications. I do believe that the CRWBY will reveal the complete story of Cinder at some point.
So I think this option is very very unlikely.
And I personally believe that Cinder is not dead. If she was, we would have seen her maiden power escapes. I assume this transfer to be instantaneous ; regardless to who the power goes we should at least have seen it escapes Cinder's body (like Amber).
Option 2 - Cinder lives through magic : well magic is part of the World of Remnant so even if no one has been seen using magic to escape death (or even resurrect), this option can't be counted out. But the thing with magic is that you don't really have to justify it... This would be too easy and again a weak move by the CRWBY to go there. But I trust them on that, I think they won't. So I consider this option to be even more unlikely as option 1 :)
Option 3 - Cinder saves herself : you said this won't happen but I disagree with you on that one. 1) Falling off a cliff is lethal in most cases but... Do we really know what is at the bottom of that specific cliff (inside a vault should I add) ? How convenient it was to put a black environment so as to make us believe the cliff is indeed very high and so that it is even more likely to die if falling and also to not being able to see what Cinder's body becomes. I believe the CRWBY did that well on purpose to hide that Cinder didn't die.
2) Being frozen is not enough as just moments before we saw Raven breaking the ice prison Cinder created on her ! Because Raven is a maiden. And Cinder also is (because as I said above, I believe she is still alive and thus still a maiden). So I don't consider the ice to be a problem. But again, who knows what is at the bottom of the cliff ? Rocks ? Water ? Lava ? Something else ? Again it was not shown.
3) The lightening Raven used only destroyed Cinder's mask and projected her into the air. The thing is, it didn't kill Cinder. So clearly not enough to justify Cinder's death for me.
In overall, there are a lot of things that we don't know because again we never saw how Cinder's fall really ended. I think the CRWBY managed to keep themselves a way out to bring back Cinder. So I consider this option to be very likely ! We could also imagine Salem's crew going back to the vault (I very much wait to see how everything will be reported to Salem and her reaction).
So in conclusion, I don't think the writers have written themselves into a corner like you said. Cinder's character has to be completed, she can't be gone. Remember how Torchwick died ? Just before his death, he revealed why he was doing all this (when answering Ruby). This was the missing part to complete Roman's character ! I personally think this was brilliant and I trust the CRWBY to handle their villains.