Yes, Cain was cursed. The "mark" placed on Cain is another one of those anomalies that is really hard to know, if it is indeed to be known.As you've concluded NO ONE can know why Cain's sacrifice was rejected.
What we DO know for sure is that he was cursed AFTER he killed Abel.
He was told that sin was crouching at the door and he must master it. Sounds like free will.
He was told the that he should "do well".
As to Hebrews 11:4
The writer, whoever he was, was not in the mind of the writer of Genesis and cannot know, as you've stated, what is not written. BY FAITH is not written anywhere in Genesis 4.
I hope you also believe Hebrews 6:4-6 if you totally trust the writer.
We are not to add to scripture.
Revelation 22:18
18 I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book;
Some have "assumed" it was a scar, or a tattoo and I have even read where some said it was that he was a black man. That is completely unbiblical.
So what the Bible does not tell us is something that we will not know.
As for Hebrews 6:4-6, yes, of course I believe those Scriptures, But.....as this is another one of the Bible’s most difficult passages to interpret, but one thing is clear—it does not teach that we can lose our salvation which is why I think you posed the question.
Not only do I understand those Scriptures NOT meaning we can lose our salvation.....with all due respect I am not going to spend any time in trying to convince you or anyone else that we can.