Luminum

Emma probably gets her rep from her unflinching use of her body to be provocative. She even mentioned once that in the Hellfire Club, she enjoyed wearing the patronizing and sexist mandated underwear and corset because it distracted men and made them more susceptible to her telepathy. It comes more from her self-maintained image of being a sexual being and the embodiment of men's desires in order to keep the upper ground in an interaction rather than anything she's actually done.

Um, no. Rachel was merely trying to come up with a metaphorical way to humiliate Emma and represent her fraudulent "good" nature. Emma verifies that Rachel's projection is only what she imagines as Emma's ultimate shame by saying that the scars on her soul are far worse than any Rachel could come up with. In that sense, she means that she is not ashamed of her attention to her physical appearance because she has stains far worse than being shallow on her soul and even with those in full knowledge, Scott loves her anyway.

So no, it's a metaphor. She's not fat, and even if she was hiding it with telepathy, there's no way her diamond form would make her thinner, and the Phoenix would have seen through her illusion because it burns through lies. ::shrug::

And highly considered a coloring error (though it was repeated in the last issues of Whedon's AXM). Basically, it's established that she can't use her telepathy in diamond form and she's immune to most telepathy while in diamond form. Martha the mutant brain doesn't count because she gives off psycho-chaff to mess up other mutants and Jean...well...Jean was using the Phoenix, so anything goes there.

As for the beginning of Morrisson's run, I imagine that parts where Emma appeared to use telepathy while in diamond form are just because he hadn't nailed down the finer points of her diamond form or more coloring errors had occurred.

Hardly, the Charles even harnessed the Phoenix himself back when he was with the Starjammers as "Bald Phoenix." The Cuckoos are strong, but they're not as skilled as Emma and nowhere as skilled or powerful as Xavier. Xavier is the strongest by far. He could take them out not only by blowing them away, but by skillfully handicapping them.