I kind of like your thinking however the flaw in this theory is that a ghost is only the spirit of a dead person. So Qui Gon, Obi-Wan, Annakin... none of them have 'cheated death', they are all very much dead, but their spirit/soul exists in the netherworld and they have learnt to communicate from this place in the form of a voice in your head or visually as a floaty ghost thing. So when Yoda says that Qui Gon has 'returned' from the netherworld, he's just means that his spirit can now be communicated with.
Also remember that the quote is in response to "Just help me save Padmé's life. I can't live without her". I'm sure it would have occurred to Palpatine that Annakin meant more than just as a ghost.
Also if you conisder this 'cheat death' dialogue along with the darth Plageuis dialogue:
Palpatine: ... He had such a knowledge of the Dark Side, he could even keep the ones he cared about...from dying.
Annakin: He could actually...save people from death?
In order to become a ghost you have to die first. Here they are specifically talking about preventing death. So I think to 'cheat death' is to be on your death bed, verging upon life extinct, and then suddenly miracuously bouncing back, using the power of the Force. Palpatine possibly did this when he saved Annakin by draining Padmes life - thus together they learn the secret.