threats being harmful to their target, the execution of threats seems to constitute an inefficiency:
also when the threats are targeted at (Pareto-dominated) inefficiency, i.e. at (conditional on) any actions other than (coordinating on) the most efficient?
As a concrete counter-example, there are productivity/self-control tools, wherewith people elect to target and/or execute threats on themselves to help elicit better behavior. The legal system is basically also collective threats that help us behave better, but is it inefficient such that we should do better without? I think the opposite, such that any one threat can not only be harmful, but also neutral or beneficial.
and a defecting equilibrium which does not.
Why doesn't this also require coordination? Also, there also seems to be a mixed equilibrium where both players randomize their strategies 50/50.
For the Chicken game, the mixed strategy equilibrium is not 50/50 but more specifically 90/10. And a mutual defection can also not constitute crash, but instead allow for further and stable repeated play, particularly if a mixed strategy equilibrium is coordinated and acted upon.