Why did Nick let Juliet be the point of contact between him and him mom?
At the time, their relationship had already gone through a lot. Nick felt like keeping secret from Juliet was hurting them, and he wanted to be more open with her, which makes sense from his perspective.
The problem is that not every secret is a relationship secret. Nick was also a Grimm and Kelly was a Grimm living on the run. Communication between them should have remained direct and limited to Nick and Kelly.
This is exactly how Kenneth was later able to take advantage of Juliet's anger and her connection to Kelly. If Juliet had never been given a direct line of communication to Kelly, Kenneth would not have had an opportunity to exploit that connection, locate Kelly, and ultimately lure her to a trap that led to her death.
A good counterexample is Sean Renard, Renard is extremely careful when it comes to information and communication. When Adalind was being moved from Vienna, Renard communicated directly with Sebastian and Meiner. He didn't rely on a middleman and he didn't share everything with Nick. In that situation, even Renard's secrecy was the correct decision. He understood that the fewer people who know sensitive information, the fewer opportunists for that information to be exploited.
What's interesting is that Nick may have learned that lesson later. In season 5 he kept the stick hidden from Adalind. That turned out to be a smart decision because Bonaparte might have found a way to learn about the stick through her. Whether though magic or simple manipulation, the risk was real.
The question is whether Nick truly learned the lesson, or whether he was just being cautious because he was still building trust with Adalind. The show never gives us enough situation to know for sure, but since Nick told Adalind about the keys and the trip to the black forest, we can assume he did learn that lesson.
That is why I think Kelly's death was not only the result of Kenneth's plan or Juliet's betrayal. It was also the result of Nick's mistake that Sean Renard probably never would have made.