In the scenario you described, where photons A and B are momentum-entangled and you measure the locality of photon B just before photon A encounters a double slit, the interference pattern observed will depend on the specific details of the experimental setup and the entanglement between the photons.
If the entanglement between photons A and B is such that the momentum of one photon is correlated with the momentum of the other, then the measurement of the momentum of photon B could provide information about the momentum of photon A. In this case, if you measure the momentum of photon B after photon A has passed through the double slit, you would be obtaining information about the momentum of photon A after it has already gone through the slits.
Whether or not the double slit interference pattern is observed would depend on the nature of the measurement and the degree of entanglement between the photons. If the momentum measurement of photon B collapses the quantum state of photon A to a well-defined momentum before it reaches the double slit, then the interference pattern may not be observed. On the other hand, if the entanglement and measurement process do not collapse the quantum state of photon A until after it passes through the slits, it is possible that the interference pattern could still be observed.
It's important to note that the details of the entanglement, the experimental setup, and the specific measurements being performed are crucial in determining the outcome. Quantum mechanics can exhibit complex and subtle behavior in entangled systems, and precise experimental configurations are necessary to study and understand these phenomena accurately.