It depends on the nature of the keyblade, if Master Xehanorts holds a similar power to the Heart Unlocker than it should come to question as the whole idea of a heartless being created easily isn't far fetched, but the heart unlocker is created from the PoH, their is a significant difference between someone unlocking their heart with their own keyblade and someone sacrificing their heart with a keyblade that would open the path to the door to darkness.
this is true. something else to keep in mind is that the heart unlocker was used twice. it was also used on maleficent and she did not turn into a heartless. all that happened was that the darkness in the core of her heart was released giving her more power and she took the form of a dragon(which it has been confirmed that she has always had to ability to turn into a dragon). afterwords she disintegrated into her robes where she was revived one year later by the memories of the good fairies.
kingdom hearts has very specific rules. however one thing that has been made clear is the heart is impossible to fully understand. though the rules have always existed there have been several EXCEPTIONS to the rules. this could be another.