Subhadra was Krushna's sister, I remember someone told me about her spreading Krushna's Maya, Arjun and Her son played a crucial role in history. Krushna couldn't let her marry Duryodhana as her role in the epic was limited to being Abhimanyu's mother, Abhimanyu's son was important too. Abhimanyu was amalgamation of Nar and Narayana from my understanding of this
If the plan was orchestrated by 4 Krushna, then everything went smoothly and they started and won the war, also explains why Draupadi was married to 5 brothers, if she was only married to one of them, then they would have pacified that brother instead of going for a war for her