I would email Zaida and see what she says about buttercream holding up down here. Even if you don't end up ordering from her, she'll be able to advise you one way or another. I know when we leave our butter out around the house down here it usually melts a little bit, so fondant may be a safer bet. Aside from the little bite with our cake cutting photo, I didn't even taste our cake our cupcakes! The night went by so fast and I totally forgot to grab one. With the excitement of the dinner, speeches, first dance, and entertaining everyone, you may not even have a moment to sit down and enjoy your cake.
I love the idea of surprise churros! We has nachos and empanadas as our midnight snack surprise.
Yes, I am worried about the cake melting but we are big foodies and I like things to taste good! I will take your advice and send her an email. I am so geeked about the churros!