Shakti can be described in a number of ways. The straight definition is that Shakti is the energy or power by which the Cosmic Spirit manifests itself. Now I will go through some hypothetical definitions and respond to each:
A power to be controlled and utilized – That is siddhi. To the extent that the individual spirit has realized a deep connection to the Cosmic Spirit, Shakti can be expressed through the Will of the magician.
A divine being to be worshipped – Yes, absolutely for sure. That’s how the connection is made and expanded.
Pure individualized self devoid of ego – You must mean pure UNindividualized self devoid of ego, otherwise it’s a contradiction, a self-contradiction, so to speak. Anyway, that’s a description of the Absolute, or Cosmic Spirit, and yes, Shakti is Cosmic Spirit made tangible to the refined and trained mind.
A Goddess and/or an intelligent and aware force – In mythology Shakti is usually thought of as a Goddess, consort of Shiva, and often worshiped as the Divine Mother as a way of going back to the Source from which we came. But let’s not get stuck on gender – Shakti can be generated and accessed through worshiping Shiva and other “male” god forms too. Shiva and Shakti are flip sides of a coin, inseparable, dual aspects of the One. Yes, Shakti is an intelligent and aware force. It is consciousness, after all.
Blind force, something which can be possessed, transferred, or lost – No, not blind force but quite the opposite, and “possessing” Shakti is just a casual manner of speaking where one might hear that such-and-so a siddha “has a lot of Shakti,” but of course the meaning is not to imply literal ownership, as it is the siddha who is a devotee of the Shakti (and he must be a good devotee if people are saying he has a lot of Shakti).
Shakti can be transferred to another person if they are properly receptive, which is the basis of an initiation lineage like the INO. It can be projected into an object as well. It is not the same as prana. Prana is life force associated with the body; Shakti is a cosmic spiritual force. Every living person experiences prana every moment, fewer experience Shakti.
While sitting for puja, the Nath subsumes his/her prana into the Shakti as an integral part of the yoga, however ultimately it must be remembered that Clinging To Life is an obstacle to spiritual liberation. Once attained, Shakti can be lost by allowing Kleshas to proliferate unchecked.