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Typically, 90 minutes for immigration at EWR is quite safe. To be on the safer side, ensure that you queue up soon after you exit the aircraft. Your seat position will also effect where in the queue you end up. Try, if possible, to get a seat towards the front of the aircraft (numerically lower seat numbers). Also, at 1400 hrs not all counters will be open. This may delay immigration a bit. Of course, it also depends on whether someone ahead of you has an immigration issue and holds a counter busy.
With regard to the second part, make sure you actually can check in till 1600 for a 1630 flight.