We do not find a son of David named "Yerimos"!
Radak: Perhaps he is one of those mentioned, just his name was changed here.
How is Machalas related to Avichayil?
'Rashi', Radak: She is not. It means v'Avichayil (he took also her for a wife 1 ). This is like "k'Cheves Aluf 2 " (Radak ? and "Shemesh Yare'ach" (Chabakuk 3:11), "Reuven Shimon" (Shemos 2:1)).
'Rashi', from Sifri Devarim 1:17: When Shmuel went to anoint a son of Yishai, and Eli'av was in front of him, he said "Ach Neged Hashem Meshicho" (Shmuel I, 16:6). He erred; really, [he sensed that] Eli'av's daughter would be queen (Rechav'am's wife). This was a punishment for answering Sha'ul "I am the seer" (ibid., 9:18); he should have said humbly 'what do you want?' Hashem said, in the future you will see, and not know what you see! You will err about Eli'av and his daughter.
Yirmeyah 11:19: Rashi (there) ? it is as if it says k'Cheves v'Aluf (a lamb and a bull).
Why did he marry descendants of Yishai?
Malbim: He wanted that his son will inherit his kingship without controversy. Therefore, he arranged that his Bechor will be from David also from his mother. His first wife did not give birth, so he married a second wife from Yishai; she gave birth (verse 19).
