The idiom "with all ... heart" means "completely" and "with all ... soul" means "with all ... being." These two phrases have similar meanings. Alternate translation: "with all their being" or "with all their energy" (See: [[rc://en/ta/man/translate/figs-idiom]] and [[rc://en/ta/man/translate/figs-doublet]])
The word "throne" is a metonym for the king who sits on the throne. The litotes "will never cease to have" can be stated positively. Alternate translation: "your descendants will never cease to be kings of Israel" or "one of your descendants will always be the king of Israel" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-litotes]])