In the thought experiment I would also likely vote pope since he seems by far the most likely candidate to win and also would not be a moral leader so bad that I wouldn't want to live in that world.

The result of your gedankenexperiment is fully Catholic world

Not actually true, I'm sure lots of educated people would make the guess that the pope is likely to win the election and vote the same way. I'm also pretty sure many non-Catholic Christians might decide he is the best pick likely to win.

I'm also pretty sure almost instantly after the calamity lots of humans would start worshipping the aliens.

and Crusade decared against the alien scum.

Unlikely to happen because of how suicidal that would be and that most Popes being intelligent people would realize this and would I think encourage the "turn the other cheek" memes to deal with the grief and outrage. However a few billion deaths might animate mankind aware of the cost in powerful and difficult to control ways.