postgres_fdw performance is slow
If you use use_remote_estimate be sure to run ANALYZE the foreign table  (I see estimations pretty close with the returned, you'd probably did it). Also, the pushdown improvements are not available in &lt;9.5 version. I also assume that you have the same table structure on the remote server either (including indexes). If a bitmap is needed due to the low cardinality, it won't use the index due to the limitations on the pushdown mechanism. You may want to reduce the amount of returned rows to force a BTREE index scan (timestamp ranges). Unfortunately, there is no clean way to avoid the SeqScan on the remote server if the filter returns +10% of the rows of the table (may vary this percentage if the planner considers that scanning the whole table is cheaper than seek reads). If you are using SSD, you will probably find useful to tweak random_page_cost).

You can use CTE to isolate the GROUP BY behavior:

WITH atable AS (
    SELECT "IncidentTypeCode"
    FROM PUBLIC ."IntterraNearRealTimeUnitReflexes300s"
    WHERE 
       ("IncidentDateTime" 
              BETWEEN '2016-05-01 00:00:00'::TIMESTAMP WITHOUT TIME ZONE 
                  AND '2016-05-02 00:00:00'::TIMESTAMP WITHOUT TIME ZONE)
)
SELECT atable."IncidentTypeCode", COUNT(atable.IncidentTypeCode) 
FROM atable
GROUP BY atable."IncidentTypeCode" 
ORDER BY atable."IncidentTypeCode";