Balors are not the final stage of mortal souls processed into demons, Demon Lords are. Balors are the weakest of the True Demons, that is demons who were created directly rather than having once been mortal.
The heavens are not connected like the hells are. This is in part because Ceridwen is the only one able to link worlds like that, and in part because several good gods do not get along. (The disagreements between Junil and Sirona are the most noteworthy.) Once a mortal soul passes into a heaven it is processed into an angel to serve that heaven's god. This is a much less efficient process than the assembly line of hell, so far fewer mortals make it to be angels than demons and their ascent is usually much slower. Without a buffer like Mulcarn's hell the good gods have no capacity to claim excess souls and wait for them to be ready to process, but are limited to accepting mortals whose deeds and faith in life already began the process of the transformation. Angels are also more specialized, focused on the virtues of their god rather than having some skill with all the vices of the evil gods like most demons have.
I am not certain, but I suspect that Sucellus might have no heaven. I think his heaven was mentioned in the original Arthendain pedia entry, but that was changed to simply having the spirit of the God of Life floating through Erebus. Nemed likely formed a world for himself before becoming mortal, but I see no evidence that either Arawn or Sucellus ever claimed it. If the heaven of life does exist, it is likely mostly empty. Lesser angels were not created until after Nemed fathered the human race, so the angels of life were created by Arawn to guard the passages through which souls would be able to return from the netherworld to Erebus, and led by Basium. Basium hates Sucellus, and most angels of life probably serve the fallen archangel rather than the upstart god. The God of Life may lack the resources to process souls into angels, or may simply have a preference for resurrecting them in still mortal forms to serve him.
It is possible for angels and demons, whether once mortal or not, to fall or rise and serve another god. Most good gods (especially Junil) don't trust former demons and are unlikely to accept them, but Sirona would always show them mercy. I would not be surprised if many of Sirona's angels and angels in training spend much of their time searching though the hells to find any souls willing to repent. Most good gods also do not trust the few Serephim who did not fall with Bhall (in part because many of those later changed their mind and are now powerful demons), but Lugus accepted them en masse. Lugus and Sirona are both accepting of former sinners, but Lugus tests them all very carefully while Sirona prefers just to hope for the best. (I don't have much to base this on, but I like to think Chalid Astrakien was once among the Emyrs, a worshiper of Ceridwen more evil than the Profanes of the Ashen Veil. We know at least once captain of the Radiant Guard was raised in that foul faith.)
