Blogging question on Book V: What is the nature of redemption available to angels within Paradise Lost?

In the end of Book V, we see Abdiel change sides from that of Satan back to that of God. This raises a serious question regarding the nature of redemption for angels. Before this incident we are under the belief that angels, once fallen, cannot be forgiven or repent. In lines 903-905 we read, "From amidst them forth he passed, Long way through hostile scorn, which he sustained Superior, nor of violence feared aught;" Here, Abdiel steps out of the camp of fallen angels back to God, and is received. We also see the question of redemption come up in Satan's self reflective speech in Book IV when he states, "But say I could repent and could obtain By act of grace my former state; how soon Would heighth recall high thoughts, how soon unsay What feigned submission swore... This knows my punisher; therefore as far From granting he, as I from begging peace:"(93-96;103-104) Satan is saying here that if he were redeemed from his actions, he would forget what he has done and rebel against God again. In this, God not redeeming him is a form of grace for him. Now, if we look back at Abdiel, we read he was, "...faithful found, Among the faithless, faithful only he; Among innumerable false, unmoved, unshaken, unseduced, unterrified His loyalty kept, his love, his zeal:"(Book V Lines 896-900) Abdiel did not fall like the other angels, but instead stood against them all, and because of that he was welcomed back into Heaven. We see that angels who fall, fall on their own accord and own choice, and would fall again if redeemed, and because they have fallen on their own devices, they are not allowed redemption, because they will fall again.