Might that be due to not all things we fancy to call sin are in fact sin?

I suppose this all gets down to what your view of sin is. If it is a failure to adhere to an arbitrarily composed list of rules, then there is no good reason why even the devil himself would not potentially be converted and once again live in peace in paradise.

If on the other hand sin is unreasonable and unreasoning hatred and enmity towards God, then it seems likely that anyone who having clearly faced and known the grace and love of God and non the less wilfully rejected it and thereafter actively fight Him will be irredeemably lost. The fallen angles presumably have had all necessary exposure to God to have such a clear understanding of who He is.

I cannot possibly agree with this. God is doing all this just so we are "revolted" by sin and stop sinning? It is the Holy Spirit dwelling within us that prompts us to live without sinning (Romans 8:9), not revolting memories. In fact, God says he himself will forget our sins.

Besides, some things set out as "sins" don't have horrible, revolting consequences, like going to church on Sunday instead of Saturday. There are millions of happy, Spirit-filled Christians going to church on Sunday.

Some who have all their lives been led by feeling, have thought that an education or a thorough knowledge of the Scriptures was of no consequence, if they only had the Spirit. But God never sends his Spirit to sanction ignorance. Those who have not knowledge, and who are so situated that it is impossible for them to obtain it, the Lord may, and does, pity and bless, and sometimes he condescends to make his strength perfect in their weakness. But he makes it the duty of such to study his word. ... {GW92 123.1}

There are hearts that the Lord has touched with His Holy Spirit. No sooner does grace begin its work in the soul, than the heart is humbled and subdued; there is no wrestling for the supremacy; pride is gone; there is such a sense of the love of Christ in giving His life for sinful beings that there is no desire to become self-exalted. The converted one sees that his Redeemer lived a life of humility, and he desires to walk in His footsteps. The missionary spirit is awakened in his heart; and, while walking humbly and circumspectly, in accordance with his faith, he can not rest until he is engaged in the work of trying to win souls to Christ. He wants every one to know the preciousness of a Saviour's love. In his work of faith and devoted labor, he will meet with strong temptations and trials, for the Lord tests all His children. If he has the root of the matter in him, he will become more and more firmly established in the truth. If Christ is abiding in his heart by faith, sin appears revolting. While he will have love, kindness, and tenderness for the scholars under his charge, he will feel that as a faithful servant he must discipline and preserve order in his class. If truth is cherished, the love of the Saviour is revealed in his words and deportment. The Word of God with him is not a dead letter; he will give not only lip service, but heart service. {TSS 25.2}

Is death a "horrible, revolting consequence" of sin? If so, then the sin itself, however mild, has just such a consequence. Whether this includes eating a piece of fruit, numbering the people of Israel, hitting a rock instead of speaking to it, turning stones into bread, or going to church on the wrong day of the week, sin has consequences. Death ranks first and foremost among these consequences.

When two options are put out in polemic against each other, generally the first one given is a thesis which the speaker does not agree with but which he speaks against. Then the second proposition which is given represents the proposed thesis to be considered by his audience.

I have already shown that death and sin are not related, since evil angels have sinned and continue to live.

At best it is an unfortunate side-effect of sin on this earth.

The point that I am making, and which you should address, is that the horrible memory of the death of Christ will not keep us from sinning.

I have already shown that death and sin are not related

In a recent SS discussion someone mentioned that Jesus died for the angels as well as humanity. No one seemed to really object or agree, but I think this raises some questions. Does the salvation provided by Christ extend to the angels as well?