What is death?

I was pondering this statement by Joseph Smith as it relates to the salvation dead.

"...the past, the present, and the future were and are, with Him, one eternal 'now'." History of the Church, 4:597.

If everything is "now" to the Lord, is anyone really dead to Him? Death seems to be only a function of time, which we know the Lord experiences differently than we do. So if all of us are existing premortally, mortally, and postmortally to the Lord, how does that relate to the idea of vicarious ordinance work for the dead?

If we do the baptism for an ancestor who died, say 100 years ago, for us that is the past and the person is dead, but for the Lord, it is now and the person is still alive. Is the ordinance, then, performed in the present to the Lord?

In the same volume of the History of the Church as the above quote, the Prophet said:

He knows the situation of both the living and the dead, and has made ample provision for their redemption, according to their several circumstances, and the laws of the kingdom of God, whether in this world, or in the world to come.

However, we know that certainly ordinances can only take place during the mortal 'circumstance' or estate. For those who did not have the covenants and ordinances done during their mortal estate those covenants can be entered into vicariously by family members who are in their mortal estate.

How marvellously complete is the Plan of Happiness of our Father in Heaven? How far reaching in it's scope? How perfect in it's application to His children?