I am not sure I understand the issue you raise, and I didn't get it last night either, so this is a flaw of mine....
Seems like Tolkien was describing unwillingness to change as a character trait, and a weakness or a flaw in elves, but are weaknesses and flaws the same thing as sin? - if you have a flaw are you then "fallen?" - in what way are they being asked to change? - and which ones aren't changing and hences "fallen"? - all of them?
I admit to quite a bit of fatigue tonight so maybe you will have to talk slowly. What exactly is the point?
I'm not sure theologically, but I think that while an unfallen creature may have flaws and faults, he will not
act on them. To do so is to sin, and therefore he is no longer unfallen. Put it like this. Maybe you have a character flaw - you have a short temper. That's fine, but if you throw a boot at someone's head and knock her out, that would be a sin.
Does that make any sense? Penny, should we ask your elder for an opinion?