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The low-pass time constant is C1*(R5||R6) which is effectively C1*R6, that's why I said R6 rather than R5 makes the low-pass. But all components play their part. You could also emphasize that R6 is required to achieve stability when C1 is present.
If the corner frequencies of both low-pass filters should be the same then differential amplifier must be designed different or C1 and C2 must be calculated regarding the input impedances of both inputs of subtractor.
R3 and R6 are part of subtractor and must not be omitted. BTW without R6 input impedance of that input will be 0 ohm and circuit will oscillate as already said.