So i've just started reading the WN translations, and in Arc 5 Chapter 17 there is a discussion between Julius and Subaru about Heinkel's relationship with Reinhard. It goes something like this :
Julius: “There was a period of time when Reinhard gave Heinkel-sama his complete, utmost obedience. That might seem natural since they’re father and son, but... that time exceeded the bounds of what it should have been.”
Subaru: “...so, to keep Heinkel from giving Reinhard an order to turn against the kingdom, Heinkel was shown favor, right?”
Julius: “Perhaps it’s even worse. Although this is still a rumor that is considered hearsay, I’ll tell you, since you’re also Reinhard’s friend, and you felt anger for his sake.”
With that troubling opening remark, Julius looked over their surroundings with a sweep. Confirming that there were no eavesdroppers, he stood close to Subaru’s side.
Then,
Julius: “The deputy commander was a suspect in the investigation of the abduction of the princess fourteen years ago.”
Could it be that Heinkel is the deputy commander and that he is affiliated of the abduction of Felt/the Princess. (I am using Felt as the princess as her age and appearances fit with the royal family.)