So, now that we've seen her work through these thoughts and dismiss them... then surely this *ship* has sailed?
I wouldn't be so quick to proclaim U.S.S. Faybles a wreck with all hands.
I'm not coming at this as a shipper myself. I like the idea of the pair being a couple, but in the main it's the emotional exploration I enjoy. The point is, I don't think I'm clutching at straws when I say that your reading of "Nah, can't be." isn't the only possible one.
I think it is equally plausible that Faye is dismissing the idea of Bubbles being interested in her. For whatever reason- she finds the idea disturbing, she thinks she isn't good enough, it's just never occurred to her that Bubbles would be interested- these are all possible reasons why Faye wouldn't consider her friend having feelings for her as a possibility.
I could even go further, and say that her expression in panel 3 looks disappointed that she feels she has to shut down the idea. But that would be tenuous.
The point is, that strip doesn't really elucidate any of Faye's feelings at all. At least not as clearly as you're suggesting. It just says in response to a word salad "nah, can't be." That's... actually pretty ambiguous.
(A hypothetical example of this:
A: "I think your friend C is attracted to you."
B: "Nah, can't be."
A: "Don't be dense, it's true, she told me so herself!"
B: "You're serious? Wow! Oh my God that's wonderful!")
It's not that I think you're definitely wrong, I just think we aren't at the stage of launching the lifeboats.