Hirah was bad company. Also the death of Shuah could be related to some act of disobedience on her part on a result of Judah's deception of Tamar. 

If anyone deceived anyone it was Judah. Deception by definition is to cause someone to believe something that is not true. He decieved her into thinking he would actually give her Shelah which says how disconnected from the things of God Judah was. Because if he knew the signifigance he would have taken this situation much more seriously. But, God is the master of this plan and he will fulfill his purposes. 

Onan was not concerned about the will of God. We know that it is important for the line of Judah to continue becuase Messiah would come through this line, but Onan did not care about the things of God. Much like his father, Judah, concerning his brother Joseph. If Onan had been concerned about the thing sof God he would not have selfishly withheld children from Tamar and would have been a respectable guy. But, instead he was concerned about himself. So, he used Tamar as an object of gratification which is evil and did not obey the command of the Lord that came through Judah, concerning giving Tamar children in order to continue the line of Judah. This is a perfect example of why it is impossible to serve God and yourself, a servant cannot not have two masters he will love one and hate the other. And obviously Onan was a servant of himself thus a servant of Satan making him an enemy to the things of God. 

I don't think the word grown has to do with him actually being old enough. I think it implies that Judah wanted his son to be great first and then marry her. Which fits into the idea that Judah thought his sons dying was Tamar's fault. But, they were the ones who did evil in the eyes of the Lord. I think it is also an important thing to highlight, that Onan was killed because he destroyed the seed that would give Tamar life because he didn't want to raise children for his brother. I think this says two things about God; 1. Tamar was important to God and her continuing the line of Judah was important to him. 2. using someone as an object of self gratification, in this context sex, is not acceptable to God.

I do wonder if destroying seed is like destroying life to God. 

I can't imagine how Jacob must have felt. He sent his son out to his brothers, I wonder if that thought tormented him. The thought and image of your baby being ripped to pieces is sad. The brother's were saw caught up in their feelings about Joseph that they did not take the Father of their brother into consideratoin. How would their Dad, mothers, and sisters respond? They hated the revelation God gave Joseph so much that they selfishly cast him away. 

If anything the errornious message that is applied to Jacob, the God used a screw up narrative, would apply to the brothers. They were not cut out from the nation of Israel but became tribes. God can make all things work together for His good. But, that does not by any means suggest that the brothers were anything next to right.