your theory makes perfect sense to me Val.
Ancient Dwarvish lore fortold that there would be seven incarnations of Durin the Deathless, the first king of Durin's line. It also fortold that Durin VII would be the last king of this line. What you are seeing in that family tree therefore, Mellie, is Thorin III Stonehelm as the last known king, and then a prediction that somewhere down the line there will be a Durin VII.
I have read somewhere that after the War of the Ring, Thorin III proclaimed himself as Durin VII but I have been unable to confirm this statement in any official Tolkien book.
In my view the : just denotes that there will be a Durin VII sometime and that he will be the last king.
Thank you Val. That answered my question. I really do appreciate it.