Perhaps these impairments of the transmission process could be offset, to some extent at least, simply by pushing the policy rate further than would otherwise have been the case.
That all monetary policies have external spillovers does not mean that they are all justified. What matters is the net spillovers. One source of spillovers is through the trade channel, the relative magnitude of demand creating versus demand switching effects. Another source of spillovers is through cross border capital flows, and their effect on financial stability elsewhere.