During the frantic fight against the wild hunt at Kaer Morhen at some point while playing as Ciri, an inventory wheel
popped up with three items equipped. I remember only one, the magic amulet, but I noticed there was no mouse pointer
shown to enable any one of the three to be selected.

Ciri doesn't have a viewable inventory. What you saw, I presume, was her power wheel -- the amulet counts as a power. They are not selectable, they are simply accessed via key shortcuts.

Maybe that is why she kept on dying fighting the Crones in Bold Mountain. I pressed the MMB and held down the Q key at times but this seemed to have no effect.