About Elishua .
According to the Metsudas Dovid on II Samuel 5:15, Elishama is the same person as Elishua.
I Chronicles 3:6 and 3:8 lists two "Elishama" sons of King David. Malbim suggests that the first Elishama was renamed Elishua when the second Elishama was born. Thus, according to this opinion, Elishua and the second Elishama remain distinct sons of David.