Although Firecom KEG254 only has Mineola listed as there only site per the FCC (I dont know why), Firecom has approx 12 or so low band sites throughout the county. The "western most sites" to my knowledge are in Floral Park, Rockville Centre/Lynbrook border and Franklin Square. Those are prob the western most Firecom sites.
PD has Tx and Rx all over the county. There are sites/repeaters in Manhasset and Elmont. Check the FCC datebase.
hearing nassau from lower manhattan depends on where the scanner antenna is and how high up it is located. if its in a window of a skyscraper, you'll prob hear alot of action. if its in a bldg on the ground floor, prob not too much. you'll hear more on UHF from ncpd than fire on low band.
dont know about the cross band repeaters, all of them are hit or miss.