There is no compelling reason to believe something has been omitted from the text. Here is what I get from the Hebrew:

So Cain spoke with his brother Abel, but as they came into a field Cain then rose up against his brother Abel and killed him.

Details:

The OP contends that the verb [אמר] is not speak, it is say, and it requires embedded dialogue to be grammatical.

This is clearly true of Genesis 18:15 (KJV), for example:

... And he said^ויאמר, Nay; but thou didst laugh.

By itself "And he said" is incomplete, requiring the text of what was said for it to make sense. However, the usage of אמר is not so narrowly confined.

Consider these examples:

And God spoke^ויאמר unto Noah, and to his sons with him, saying^לאמר, And I, behold, I establish my covenant with you, and with your seed after you;
-- Genesis 9:8-9 (KJV)

And it came to pass at that time, that Abimelech and Phichol the chief captain of his host spoke^ויאמר unto Abraham, saying^לאמר, God is with thee in all that thou doest:
-- Genesis 21:22 (KJV)

And Shechem spoke^ויאמר unto his father Hamor, saying^לאמר, Get me this damsel to wife.

In each of these instances אמר stands in an independent clause (complete thought), followed by a dependent clause לאמר + the dialogue. Rendering these examples as "<person> spoke unto <person>, saying, ..." reflects this, whereas "<person> said unto <person>, saying, ..." does not.

1. Exodus 1:15-16 further evidences this:

¹⁵And the king of Egypt spake^ויאמר to the Hebrew midwives, of which the name of the one was Shiphrah, and the name of the other Puah: ¹⁶And he said^ויאמר, When ye do the office of a midwife to the Hebrew women, and see them upon the stools; if it be a son, then ye shall kill him: but if it be a daughter, then she shall live.

Here אמר stands in two independent clauses. While the second would be fine as "And he said, ...", giving the first as "The king of Egypt said to ... Shiphrah and ... Puah:", would fail the OP's requirement of needing embedded dialog to be grammatical. Clearly, the first is rightly given as, "The king of Egypt spoke to ... Shiphrah and ... Puah:"

1. Then there is Exodus 19:25 (KJV):

So Moses went down unto the people, and spoke^ויאמר unto them.

אמר stands here in an independent clause, and there is no dialogue for Moses within cooee. Again, giving this verse as, "So Moses went down unto the people, and said unto them", would fail the OP's requirement of needing embedded dialog to be grammatical.

###Conclusion How does all this play out for Genesis 4:8?

Since the MT does not preserve any dialogue for Cain, then it can only be rendered as an independent clause, i.e. "Cain spoke to/with his brother Abel ...", which I have shown is not grammatically unsound. Therefore the OP's claim that the verb [אמר] is not speak, it is say, and it requires embedded dialogue to be grammatical. is invalid.

Is the MT corrupt? The OP seems to believe so, but whether it is corrupt by omission or others are corrupt by inclusion, who can tell?. When all is said and done though, if people can't discern the nature of God and what pleases Him and the nature of Man from what has been preserved in the MT, then they are unlikely to ever do so.

¹¹For this commandment which I command thee this day, it is not hidden from thee, neither is it far off. ¹²It is not in heaven, that thou shouldest say, Who shall go up for us to heaven, and bring it unto us, that we may hear it, and do it? ¹³Neither is it beyond the sea, that thou shouldest say, Who shall go over the sea for us, and bring it unto us, that we may hear it, and do it? 14 But the word is very nigh unto thee, in thy mouth, and in thy heart, that thou mayest do it.
-- Deuteronomy 30:11-14 (KJV)