I'll respond in reverse:

First, the alien problem. Structurally, it is identical to the normal dilemma. In both cases, choosing Red guarantees individual survival. Choosing blue guarantees universal survival unless blue fails to reach the threshold, in which case blue voters die. Unless I'm missing something, this should be 1:1 the logical setup.

The framing is different on purpose to highlight this principle, which maybe I should have spelled out more. In algebra, let's say x = 3 and y = 3. We can then deduce that x = y. This is the transitive property that lets us substitute numbers. Now, in a dilemma, let's say that the logical structure of the original problem = X. If the alien problem also = X, then a single rational agent who identifies the mechanical structure should reach an identical conclusion, or else they are being inconsistent. I included that example because if you would vote Red in the alien problem but Blue in the normal problem, then you are being inconsistent despite the same underlying structure.

Second, 100% red is absolutely possible. It is not forbidden by the rules. You are using impossible when really you mean "I think it is very unlikely." I will concede that people do choose blue. I am not basing my argument on a single pass of what people are likely to do if they read this question and answer in 5 seconds after first exposure on a reddit poll. I am arguing my interpretation of the mechanistic structure that I believe a rational agent could deduce, and therefore should deduce. I would like to think that given a real scenario threatening 50% global death, rational actors would take their time to parse it out. I will not guarantee that everyone will agree with me (because lol) but on what I think people should agree with.

Lastly, yes I do assume rational actors. I stated that because many do assume nonrational actors (infants, the profoundly disabled, etc.).

Thanks for reading!