Isn't the katra the Vulcan "soul" per say?
Probably not in the most conventional sense. After all, Spock donated his to McCoy and still was able to save the ship and have a nice death scene afterwards. Not a "soulless" performance at all!
Probably one can dump a copy of
one's katra contents through touch telepathy to some extracorporeal receptacle such as a fellow humanoid or a katric ark when one wants to. Doesn't mean the katra inside one's noggin would be diminished by the act.
Probably one's katra also "grows" through life, so that the katras of two adults would be in conflict if crammed in the same head for long (as we see with McCoy or Archer), but the katra of an adult could easily displace that of an infant (as we probably see with Spock-from-McCoy's-head and Spock's-body-reborn-through-Genesis).