Tinwen &
GalaGirl have certainly covered all the points.

Definitely worth re-reading.
HoneysuckleRose, the one thing that leapt out at me is that you & the SO already believe that you're "miserably love sick at this point." That's NOT good.
Most people are
neophobes, fearful of taking risks because... well, it might not work out & maybe they'd feel foolish. I'm the other way 'round: being a bit of a klutz anyway, life has a way of tripping me up (like Sideshow Bob in a rake factory

), so I figure I'm just gonna make mistakes, & often feel foolish despite my best efforts, so why not face it head-on & have fun?
Once, we decided to let a friend of a friend move in with us. She was living out in the "outer ring" suburbs of Minneapolis, with no car, no bus service, & a two-mile walk from the nearest (low-paying) job prospects. She was cute, smart, & funny, & made it VERY clear that she only wanted to get her life together & move along. Stating this was clearly difficult for her, & I admired her forthrightness.
Oddly perhaps, I wasn't attracted to her in That Way, so agreeing to this was no problem at all. She turned out to be an ideal roommate, & we lived happily together.
And she stayed years longer than she initially intended, & we were in love before we were sexual partners. (Oh, & in those years she worked her way up from mail-cart pusher to data-center manager for American Express.

)
One recurring hallmark of baseless unicorn-hunting behavior is the sense of "gotta have it ALL, right damn NOW." No dating, no courtship, no building, just plug-&-play from the first fancy box that comes within reach.
Therefore,
HR, I'll be blunt. My thought is that if you stifle your stupid little Romantic fluttery feelings (not picking on you; I've had 'em, & that's my estimate), it'll make all three of you neurotic & weird, sooner rather than later. This scenario will NOT end well.
If you tell her about the baseless sex-fantasy crush, she may bolt & you'll never hear from her again. Short of her pulling a gun, that's about as bad as it'll get.
Or, she'll read you her personal Riot Act of how you're NOT gonna be allowed to infringe her boundaries, & she intends to move along as soon as she gets her life together... but maybe with respect for how you dealt with her straightforwardly AND respected her boundaries. To me, that sounds like an ACTUAL basis for eventual courtship.
Impressed by your honesty, will she acquiesce from the beginning? Probably NO WAY IN HELL. And if you press her, consciously or not, you're terrible friends. If you were willing to sacrifice friendship in hopes of squeezing her, then I'd contend it wasn't there in the first place.