No she hasn't. She's just got nothing to summon anymore. That's different to losing the power to summon and the power to send. Though she might be out of practice, the inherent ability to manipulate pyreflies a summoner needs to perform a Sending likely remains.
As has already been stated many times by now, Maechen cannot have been from DZ for two reasons: 1) There is physical evidence of Maechen existing in Real Zanarkand (the Black Mage sphere). 2) He knows far too much about Spira to be from the Dream. Remember the Dream is isolated from Spira, basing its original construction from the Real Zanarkand, but otherwise passed no information from Spira to the Dream, ie, No one in DZ ever knew about Bevelle, the Ronso or even about Aeons.
Though I would like to challenge that an unsent dream citizen would disappear with the rest of the city. I don't believe they would, and would exist like normal unsents, entities derived purely by pyreflies based on the soul that permeated them in life. Consider that Tidus' Mother is seen in the Farplane, implying that after death, the pyreflies themselves can persist an entity (as an unsent or as a farplane apparition) and no longer need the summoning to keep them together, and we do hear from (SPOILER)Jecht in the final battles of X-2, after the dream had ended, who was a dream citizen.




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