I don’t. Any more than I can explain angels at all (fallen or otherwise). The Bible makes reference to them in speaking of temptations or servants and such. But that isn’t so much explanation as reference. So I’m content to be aware enough of those references to try not to misuse them or abuse them. “The devil made me do it” might be one such abuse we occasionally hear today, though usually flippantly. “Principalities and powers” are another reference we sometimes see partly personified in the Bible. We have our modern categories for some of those things too that fulfill their same age-old roles of temptation or coercion. So I guess it doesn’t disturb me over much that these things aren’t “explained” as some might like because it doesn’t seem to have disturbed the original biblical authors. I’m content to leave that category of beings be what they are beyond my knowledge of how to categorize them or even to know if they exist at all as often popularly imagined. As to when “the fall” happened - that would be even farther back into the obscuring mists already inherent in the questions raised above. It may quite well turn out to be a meaningless question to seek after a historical time-line kind of answer to such questions.
As to why God might withhold forgiveness - I don’t think God does. Or I guess another way to see that is that I think God forgives any agent that is capable of repentance and
seeking accepting that forgiveness. God would be less than loving - merciful - or perfect, if he didn’t. So if something / someone remains unforgiven into perpetuity, it must be because they have kept themselves unable to receive any such forgiveness.