Yes! I just finished it, and I did not find it heartbreaking in the least. In tone, it's most like the first part (ETA: chronologically earliest, as recounted by Julie in her written account of Maddie, not the first actual pages of the book) of Code Name Verity, when Maddie (Sir Not Appearing in This Book) and Julie are making friends and hanging out and having a good time, before everything goes south.

Re the spoiler, I saw it developing, and I've been so conditioned by every book ever that I was thinking, "The character doesn't realize what's going on, and the author's never going to go there, but the ambiguity's going to be great fodder for fanfic authors." Then she WENT THERE. And then I kept waiting for her to go back on it, decide it was a phase, etc. AND SHE DIDN'T.

I foresee glorious Yuletide treasures coming out of this!