Questions, lovely.
Well
@Dedtoo a mortal ascending into godhood isnt unusual as such, some gods decide on a mortal successor and have them ascend (a version of becoming a god over an already existing race, regarding your gods age).
But if you want a mortal to ascend through his own power... possibly... But remember that gods come in many shapes and sizes.
Dont mess with the dead. Dont mix the dead and the living. Dont tread upon planes you should never reach.
And now that I have said the standard warnings...
The plane of the dead is a place filled with those that have died in this world, some may retain their reason, but above all it is a place of tranquil nothingness. The dead dont want, they dont need, they dont seek, they are.
But theoretically, they could make a realm of their own, just as some gods have a realm of their own, sure. A bunch of mortal magicians could make a realm of their own, though it wouldnt be big. A plane is something entirely different from a realm, as a plane is a different frequency of the world, while a realm is either an enclosed and unconnected space, or simply an area where your influence is greatly enhanced.
And more or less anything could act as a race's deity, the question is who gains anything from it.