Here’s my thought: it comes down to a question of evolution vs. development.
Analogy: Will a spider monkey ever evolve into a gorilla? Common ancestor, but now different lines of development.

I think we are proving our essential, eternal core both to ourselves and to the universe. Not to God, for he knows our hearts, our potential, and our destiny from beginning to end.

I suspect that God with perfect omniscience selected Jehovah as his Firstborn in the spirit, Lucifer as a star of the morning, and the absolutely perfect number of Celestial intelligences, Terrestrial intelligences, Telestial intelligences and Lost intelligences to create a system of maximum happiness for each one after this probationary state.

As within the confines of two lines emanating from a point, eternal progression is possible within the limitations of a Telestial kingdom, and even in outer darkness. I don’t think the monkeys need or will ever evolve into gorillas, worlds without end.

If I’m wrong (it has happened several times before), then I’ll be glad and welcome the prodigals in with a party. But, for the present, looking at every soul’s free exercise of their agency as a success is helping me to be more patient and loving at home, at Church and even with politicians.

Our job, as a people, is to prepare the world for the 2nd Coming and millennial reign of the Savior. When a son of perdition receives my testimony, receives the witness of the Spirit, hates it and kills me — that’s a success! We are both prepared for the 2nd coming, and God’s honor/power are elevated. (D&C 29:36)

More commonly, when a member of our ward fails to do his HTing, or preaches careless false doctrine to children, I’m anxious but not angry any more.

My goal is to be holy, (whole, healthy, balanced), for God is holy. My prayer is for the pure love of Christ that never fails. This quest seems to have brought me to seeing Telestial and Perditious behaviours as successes. Temporarily painful, but eternally joyous for all. (Moses 7:39 [24-44])

It would seem there is room for the progression of all that will progress. On the other hand Brigham made it pretty plain the Sons of Perdition would return to the elements they were made from so I don’t know about those.

I also find it hard to believe Father would not let his children progress if they find it themselves to do so.

My thought process has come about this way. D&C 76 (several instances) seems to indicate a seperation between kingdoms. For example, speaking of those in the Telestial Kingdom, we are told that where the Father and the Son are, they cannot come (vs 112). I have a wayward son, 22 years old, returned missionary, who has had his name removed. I cannot fathom that I would ever give up on him – ever. And if I, an imperfect mortal with flaws and limitations, have that level of persistent dedication to a wayward child, would God be any less inclined? Would He be satisfied, knowing that He had spiritual creations who would never again see Him or receive a fulness?

I don’t know the answers to that question, and from what I’m reading, I don’t think any of us do. But my own heart tells me that I would hope for the possibility of eternal progression for all, even those among us who seem to be lost. My personal view is that, at least on some level, it must be so.

Love it!