I don't find the Fieshi letter particularly convincing, I have to say, but when discussing theories of Ed's survival, it's pretty much a red herring. Far more compelling is a letter written by William Melton, archbishop of York, to his cousin Simon Swanland, merchant of London (he became mayor in autumn 1329). The letter was written on 13 January in either 1329 or 1330, and says 'my liege lord Edward of Caernarfon is alive, and in good health of body in a safe place.'
This isn't necessarily proof that Ed was alive, only that Melton thought he was - but Melton knew Mortimer and Isabella well, and obviously had good reason to doubt that they'd had Ed killed. Melton was in his 50s in 1330, a highly experienced and astute man with an excellent reputation among his contemporaries and historians. It's difficult to dismiss him as a gullible fool because he thought Ed was alive, as the earl of Kent often has been (beheaded 1330 for plotting to free Edward). In fact, there's ample evidence that Kent was anything but a fool - he's been assumed to be one by historians convinced that Ed was dead in 1330, and therefore convinced that if Kent thought he was alive, he must have been an idiot.
I'm not sure that Mort and Isa would have wanted Ed killed. We're looking at it with hundreds of years of hindsight, knowing that that's what happened to later deposed kings of England - but in 1327 it had never happened. Ed II was the only deposed king of England to be succeeded by his son, and by the connivance of his wife, and to me that makes a big difference. I'm no fan of Isa and Mort (obviously...

) but I don't think they had Edward killed. I think there's a good chance that his death was faked, which would protect them against his possible restoration (lots of people were planning to free Ed in 1327) without having to commit murder and regicide and without Isa having to be responsible for the murder of her lord and husband - a terrible crime.
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"Sans lui n'estoit rien fait, et par lui estoit tous fait, et le creoit li rois plus que tout le monde." Without him nothing is done and through him everything is done, and the king trusts him more than any other: Hugh Despenser the Younger and Edward II