Based on your additional replies:
It sounds like Leela knows how you feel, and has always known how you felt. And it sounds as though she reciprocates this to at least some degree.....although one then wonders why benefits would have been a bad idea before the other boyfriend came into the picture.........and, yet, she has chosen. And, yet, you are getting enough of a mixed message to wonder if you should try to be with her anyway.
It sounds to me like one of two scenarios is plausible: (1) She doesn't actually want to go through with the marriage, but doesn't want to take responsibility for that decision (i.e., she wants you to "convince her" so that if it turns out to be a bad decision, she can blame you instead of her) or (2) On the balance of things, she's just more into him and less into you than you'd like.
Your description of your relationship with Leela also makes me wonder whether she is as honest with herself, or with you, as you believe.
This modifies my advice somewhat:




or get off the pot.
Yes, still have an honest conversation with Leela. Yes, still have an honest conversation with Kaylee. But those conversations should be more along the lines of:
TO LEELA: "I know you're engaged to Tom Baker, but we also both know how I feel about you. And if you want to be with Tom Baker, I'm going to wish you both the best. But I need to move on. I can't be your fallback if things with Tom Baker don't work out."
TO KAYLEE:
Option 1: "I've had a crush on LEELA forever, and we're going to try to make it work. But, if LEELA wasn't in the picture, I would have been very interested in you, and I am really honoured that you even considered me. I hope we can still be friends."
Option 2: "I've had a crush on LEELA forever, but she's going to go marry Tom Baker, and I'm still working through exactly how I feel about that. So I'm probably going to be a bit of a prick for the next few months. So, if you want to wait and try in a few months, I'll still be here. But if you want to make a go of it now, I'm willing to try. Either way, I'll try not to act like a prick, and I'll take responsibility for it when I do."
CONCLUSION
If you can have a deep and meaningful relationship with Leela, you can also have a deep and meaningful relationship with someone else. What the heart wants isn't rational, but it is conditioned, and it can change with new input.
Perhaps the most important benefit of rational thinking is that we can sort through our (irrational) desires and put them in some sort of order of importance.
If my current partner chose to end our relationship, I would be devastated. Right now, I can honestly say there is no one in the world I would rather be with. But, if she did decide to leave, I wouldn't be crippled for life, and I know there would always be a chance of being as happy with someone else.
You need to judge what you have on the basis of what it is, not on the basis of some "something else" that you may one day have, or can imagine having. The grass may always look greener elsewhere, but grass is grass, and none of it is perfect.
Good luck.
RC