According to Unruh there is both which-way information in the coherent setup and one might infer existent destructive interference at path 4 if the destructive interference is not measured. Thus according to Unruh one can consistently infer the destructive interference, but not measure it. The antirealist logic goes by extrapolation from single-path setups to coherent double-path setup like this: verify that closing path 2 leads the photon to emerge always at path 6. This means that the photon path 6 is correlated with passage along path 1. Similarly one closes path 1 and verifies that photon passage along path 2 is correlated with emerging photon at path 5. Then according to Unruh the which-way information is guaranteed by the linearity of QM, and only trying to measure the interference by putting obstacle at path 4 destroys the which-way information. In other words one cannot measure both which-way information and destructive interference, yet one can infer the interference provided that it is not measured. This however implies that BS3 can distinguish the past of branch vectors coming along path 3, which is inconsistent with the postulates of standard QM.
