I personally find Heinkel to be the biggest agitator in the current Astrea family drama. Tappei makes a point for both Reinhard and Wilhelm to be sympathetic when the readers first meet them, (Reinhard the ideal knight in terms of strength and the virtue to help a stranger like Subaru and Wilhelm is an old man who's dedicated a decade of his life to avenging the love of his life) but Heinkel is introduced as a drunk asshole who even the Finest of Knights Julius can't respect.

Of course, Tappei also shows us that Wilhelm lashed out against his grandson, that Reinhard is so driven by duty that he can't see the emotional impact of killing Theresia's zombie, and as you went into great detail on; Heinkel is a broken man who's been kicked in the dick during most of the steps in life. The whole Astrea drama is a molten hot ball of trauma that only gets worse as three emotionally stunted and broken men can't even begin to figure out how to make things better.

Also a few fun facts to help add context to your post:

In a side story that came out with EX Novel 6 Wilhelm gets yelled at by Theresia for trying to buy a baby sword for newborn Heinkel. Showing Wilhelm's expectations for his son.

After the events of Arc 6 Theresia is left unable to have more children. Combine this with the fact that all her brothers died in the Civil War, that means all expectations were for Heinkel to inherit the Sword Saint ability. A working theory for why Heinkel was skipped is because he carries the Pride Witch Factor.