User blog comment:IamJakuhoRaikoben/Chapter 393: Silver Memories, Review./@comment-77.195.192.49-20140717144937/@comment-7418318-20140718194330

There are story settings where Necromancy involved enslaving souls and putting them in dead bodies, while other settings have the dead bodies animated purely by magic, without any mind or soul... or mixed stuff where soul-less undead are the novice stuff and putting enslaved souls in bodies is the mark of advanced necromancers.

So unless we get a better (or rather ANY explanation) about how necromancy works in Fairy Tail, we really can't judge anything.

From Roubaul, Mavis, Bickslow and Chicko=C=Hammit we already know that souls can remain in the world of the living and from Franmalth we know that souls can be enslaved. So it isn't impossible to find scenarios in which Keith could actually have captured both Silver's soul and body... or it could just be the momeries and personality imprints left in the brain... we don't really have any way to tell yet.