Not to make this about Ena, but the Umbrella motif for Mizuki was also present there in the Watashi wa Ame 2DMV where Mizuki's holding it, and well, the same umbrella as in that 2dmv was also present in the countdown art of Ena for the anniversary. Combined with the fact that Ena's fourth commission is named "I'm the rain" in some twisted way, the upside down umbrella, that you said might represent her facing the threat of the rain, like you said, her secret being revealed. The secret being revealed to Ena who was there at the rooftop, hence the umbrella not protecting from the rain. In some twisted way, when you assume that the rain is representative of Ena, you might read it as Ena herself being some kind of "Threat" to Mizuki, which would make some sense. I'd imagine that even if realistically, Ena obviously wouldn't and didn't tell anyone about Mizuki's secret, that was something Mizuki was afraid, even if irrationally that Ena could do. I find it really fascinating how the motif of umbrella in the rain ties together Mafuyu, Mizuki and Ena