I said Cain's offering was inferior. This can be seen in the text (Genesis 4:4), Abel brought the first and fattest of his flock,
I think you're making an assumption that the offering brought in this chapter is for atonement. However the text clearly states that offerings are not needed for atonement yet. If one does well, one is accepted (Genesis 4:7).
OK, ok You don't accept the Talmud. I'll not use it as a source in this conversation with you. The important point is that a Paschal Lamb offering is of lesser sanctity and does not bring atonement. It's detailed clearly in Leviticus 7:1. The burnt offering is "most holy". If someone told you that the Paschal lamb offering is most holy and brings atonement, they were misinformed.
Why do I think it's a warning? That's literally in the text. Genesis 4:15.
I also think you're misinterpreting the verse in Ezekiel 9:4. Those people were were marked because they recognized the abomination. They were righteous. This is what the mark meant. Not salvation righteousness.
Besides, you admit that the two signs were different. In Genesis it's Aleph-Vav-Tav. In Ezekiel, it's just Tav-Vav. So trying to equate the two as meaning the same thing is a bit of a stretch.
The offerings were not done for atonement. They were done to foreshadow the Messiah who would atone for the sins of people.