There is much about the nature of angels that will probably allude us until we are able to see as we are seen. Even scripture only goes so far in explaining their nature and work. Much about them falls into the realm of the secret things that have not been revealed to us (Deuteronomy 29:29). However, let me see if I can make some observations.
You mention that angels neither have a sin nature nor a free will. I will agree that they do not have a sin nature. However, it seems clear that they have a free will of some sort. They can decide to rebel against their Maker. Evidently, they are not bound to serve Him. However, there seems to be no plan for the redemption of fallen angels. The revelation they have been given is so great that no provision is made for their rejection of God's glory. So, although they have no sin nature, they can certainly choose to turn from obedience to the Lord.
Revelation 12:4 refers to the tail of the dragon (Satan) drawing a third part of the stars of heaven (probably the angels) with him when they were cast to the earth. These angels do not seem to be the same as those that are in chains. Those in chains went further than rebellion and disobedience. They perverted the very nature of God's creation.
I cannot say if angels can or cannot sin now or how they will be changed in the future. This is God's domain. I can only assume that there will not be a continual dropping out of heaven by angels who choose to sin. It certainly seems that such times of rebellion only occur on very rare instances. Surely , those who remain with the Lord will have made their full commitment to Him and will be preserved by Him for eternity. I just do not have scriptural proof for my conclusions.
