Adri'el's wife was Meirav, and not Michal! And Michal did not have a son until the day she died!
Rashi, citing Sanhedrin 19b: Michal raised them, therefore they are called on her name. If Ploni raises an orphan in his house, it is as if he gave birth to him, and he is called on Ploni's name. Radak - we find "Yulad Ben l'Na'ami" (Rus 4:17). It says "Elu Toldos Moshe" (Bamidbar 3:1), and it lists only Bnei Aharon - because Moshe raised 1 them and taught them, they are considered his descendants.
Midrash Agadah, Rashi, Ramban (all on this verse) and others say only because he taught them. I did not find anyone else say that Moshe raised them. (PF)
Why were these ones punished for Sha'ul's sin?
Radak (1) #1: Perhaps these descendants were among those who killed the Giv'onim.
Radak (1) #2: These were not among those who killed the Giv'onim; they were minors at the time, for Sha'ul ruled for only two years, and Meirav was given to Adri'el at the time that he thought to give her to David. 1 This punishment was bi'Ydei Shamayim.
Radak: It is difficult to say that Adri'el married her before Sha'ul became king, and he thought to force him to divorce her, and give her to David!
