Descriptions of the tattoo(s) in the tags save lives btw
I don't think eddie has had a crush on chrissy since 8th grade, per se. Granted i think it depends on what actually happened at the talent show - did they compliment each other in passing? did they talk a little bit while they were waiting to go onstage? did they hang out at rehearsals? - but my guess is that she was nice to eddie, he didnt ever really have anyone be nice to him before, especially not a girl, and so he had a little bit of a crush on her in 8th grade.
But as he entered high school and found out there were Girls, and she wasnt at school with him, he moved on, but he always had a soft spot for her bc hes just a little bit sentimental. And he remembers the talent show w fondness and remembers HER w fondness and remembers his crush w (probably slightly embarrassed) fondness.
Chrissy, on the other hand, never had a crush on him exactly (or maybe she did in 6th grade bc he was An Older Boy Paying Her Attention, which to a 12 y.o is like the epitome of romance), but when she got into high school and saw Eddie The Freak, she was intrigued. I think she was fascinated by him (from afar) and probably thought he was handsome in an odd way, and Jason noticed her fascination to a degree which is why he was super suspicious of Eddie and partly why he hated him so much.
My guess is that in the forest when she remembered him, she also remembered her crush, and realized that hes still just as sweet as he always was, and that combined w his blatant flirting and fascinating personality and unconventional handsomeness and the fact that He Noticed Her Suffering And Cared, and thats when she started feeling the inklings of a crush.
Had she lived, my guess is that what gvd meant when she said that chrissy would have left jason for eddie, it would have been bc she realized that the things she liked about eddie were things that jason was mostly missing, and that eddie didnt have any of the qualities that she didnt like about jason, and in the grand scheme of things, an unfulfilling relationship is a waste of time when she might die in an apocalypse