I played around with the temperature and current to recover the beat (Attachment 3):

south laser temp: 9.373 kOhm, north laser temp: 9.602 kOhm

The low frequency oscillation still remains.

I decided to re-organize the fiber to make the north and south paths more evident visually. In the process, probably because I could not quite see the fiber coupler and was inserting it incorrectly or I somehow damaged the fiber coupler, the north coupler on the ADAFCB4 did not mode-match adequately so I replaced it with a single coupler borrowed from the west path in Johannes' setup. Now both paths at the output of the fiber show ~2 mW power and the beat is at around -15 dBm. (Attachment 1).

I've replaced the south path fiber to free-space coupler mount and post with an adjustable one to get the beam height to 4 inches. (Attachment 2)