RE: It was lawful for the King, and for him only, to marry his predecessor’s wives.
Well, there were non-Kings who had more than one wife.
Just to name two...
Jacob had two wives - Leah and Rachel.
Elkanah had two wives, Penninah and Hannah ( who was the mother of the prophet Samuel ).
It’s not lawful according to biblical law...
The post only addressed King David, that’s what I spoke to.