No, you believe carefully selected proof-texts.
Gen 4:6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
Cain could have done well, and if so he would have been accepted. How can that be true if Calvinism is true? We know from NT scripture that Cain was lost. If so, in the Calvinist view he was unelect. So why would God speak to Cain as if he could do well if he could not? Is God confused? Did God forget that he had already determined to pass over Cain and that he would never be able to do well and give an acceptable sacrifice?
And why did God tell Cain he would rule over sin? This makes no sense in Calvinism if Cain was unelect.
Much of scripture makes absolutely no sense if Calvinism is true, but most Calvinists ignore these scriptures.
First, the context and then answers to some of the questions.
"And in process of time it came to pass, that Cain (a farmer with a good crop from working hard) (proudly) brought of the fruit of the ground an offering unto the LORD. And Abel, he also brought of the first-lings of his flock and of the fat thereof.
And the LORD had respect unto Abel and to his offering: But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell (he pouted, stomped his feet, crossed his arms, and filled his heart with hate).
And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen? If thou do well (go get the right stuff to offer, Cain), shalt thou not be accepted (I would not disrespect you)? and if thou do not well (keep that rebellious spirit - stiff necked), sin lies at the door (you will come to no good).
(But Cain, if you bring the right offering and the right attitude, I will certainly accept you) And unto thee shall be his (Abel's) desire, and thou shalt rule over him (because you are the oldest)."
Words in () are my own and not to be taken as adding to the Scriptures but as clarification of the setting and conversation.
Your questions:
So why would God speak to Cain as if he could do well if he could not? Because God did give Cain a choice, but Cain, who only heard with the natural ears and not from being given quickened new nature hearing, rejected God's suggestion that he bring the appropriate offering.
Is God confused? Nope - consistent
Did God forget that he had already determined to pass over Cain and that he would never be able to do well and give an acceptable sacrifice? No, but just like pharaoh, Cains heart hardened and when God spoke he hardened even more. It is God that chooses who's life will be endowed by the quickening of the Holy Spirit, not man.
And why did God tell Cain he would rule over sin? As I showed above, the conversation is about Abel and Cain being the oldest.
This little story aligns just fine in the Calvinistic thinking when taken in context.