Two families

Is it reasonable to assume that Cain's descendants followed his lifestyle and Seth's children followed him?

If so could these distinctions be made....

Chapter 4
1. Cain's descendants lived in cities or in tents (vs. 17,20)
2. They raised cattle, not for worship but for eating (vs. 20)
3. They were very intelligent and technologically advanced (vs. 22)
4. They enjoyed art, music and culture (vs. 21)
5. They accepted polygamy and other moral reductions as progressive (vs. 19)
6. Murder and crime increased (vs. 23)
7. Their women dressed immodestly (Gen 6:2)

Chapter 5
1. Seth's descendants were Christians who began a movement (church) of revival and reformation (vs. 26)
2. They lived extremely long lives (912, 905, 910, 895, 962, 969, 777).
3. They walked with God - They didn't just call themselves Christian but actively lived for Jesus (5:22, 6:9)
4. They were also very intelligent and technologically advanced (Gen 6:13-16)
Also, assuming they obeyed all God's instructions and laws:
5.They were vegan (Gen 1:29)
6. They accepted only one wife (Gen 2:24)
7. They kept the Sabbath (Gen 2:1-3) and other religious ordinances (Gen 4:4)
8. They understood accepted by faith the atoning sacrifice of Jesus Christ and were born again by the Holy Spirit (Gen 3:15) (Gen 6:8,9)
9. The men were led away from God through lust (Gen 6:2)

Chapter 6
Now we have the third race of men resulting from the intermingling of Cain and Seth's descendants (vs. 2-4)
1. These had a form of Godliness but it was in name only (called sons of God in vs. 2) meaning they still part of the church but nominal Christians
2. Their lives were short, only about 120 years (vs.3)
3. They were of impressive physical stature and were famous for their accomplishments (vs. 4)
4. They were extremely wicked and morally depraved (vs. 5)
5. They were extremely violent (vs. 11)
6. They didn't follow God's commands (vs. 12) meaning they despised His instructions to have one wife, be vegan, and keep the Sabbath
7. They were destroyed with the unbelievers

I found it very interesting that under chapter 5 you mention 5. they were vegan. it seems to be an important fact you you. yet if you read. Genesis 9:3 "Every moving thing that liveth shall be meat for you: even as the green herb i given you all things," he continuous to say in Genesis 9:4 But Flesh with the life thereof, Which is the blood thereof,shall ye not eat". 
So cook it well before you eat it

Anything that is instruction on the will of God is important to me and every believer. Tell me, of Genesis 9:3 and Genesis 1:29 which is a command and which is permission only?

I cant see where it says permission, but if it does it does not matter. God said you can do it. When you asked your parents when you where young  to go play outside with your friends that was a permission if your parents said yes you would go. Is not God your Father? Command is for instruction that God gave. Permission is when you ask God. If He says yes so be it. God could of said no...

Do you want to serve God to the fullest extent of your capacity or do just enough to get into heaven? If your attitude is to do just enough, you won't get into heaven because the requirement for every believer is to take up his cross and surrender himself completely. Also, I was remarking on the lifestyle attributes of the descendants of Seth. Gen 9:3 is a later permission given to Noah for a specific reason. The corruption of the world before the flood included corruption of the lifestyle principles God had originally given to man for his ultimate good - Sabbath keeping, diet and marriage.

Did God allow meat to be eaten?

His desire for our lifestyle is found in Gen 1. He allowed meat in Gen 9. That does not make it His will, only His permission. Since you are trying to justify meat eating today, the question must be asked is it necessary as it was for Noah or is it possible for you to live healthily eating the original diet ordained by God? If it is possible, why don't you want to surrender your personal desires and submit to God's desire for you?

so God has no free will.?

Are you Greek?

As Soon as man sinned God gave as what we deserved.