I'm gonna read this at some point, but if I had to guess, the childhood friend loses, because childhood friends are usually best girls and best girls never win.
Applying anime/manga knowledge to SVTFOE with respect to childhood friends, no Earth girl (like Janna or Jackie) can win since they all have known Marco for a long time. As for Star, no Mewni dimension boy (Tom) can win with Star, since they've known Star for a long time.
Starco confirmed. Thanks Japan. At least we have this canon romance of the side/background characters, who will win after a long fought race war