People seem to have spoken, tap to cycle and hold to cast option (or something along those lines) appears to be the majority preference here and I do agree that sometimes straight-forward solutions work best.
As in the title, Inaros and Inaros Prime seem unable to activate "Last Gasp" when in the downed (sarcophagus) state. Happens in all kinds of missions, doesn't seem to be triggered by anything in particular.