I checked the temp around 11:30pm after light been out for awhile and temp was at 78.6.I agree that 79 is not any reason for coral loss or undue stress.
Flow is very important in any marine tank and plenty is needed.
Lowering your heater will do nothing as it being set on 75 means it will not kick on until the water temperature gets below 75 which is a bit cold and will allow your tank to drop dramatically from 79
Your home temperature will play a large part on your system temperature.
Having tank near a window or not well insulated wall in your home can allow it to be sitting in a warmer part of the home which will keep temperature of the water closer to upper limits.
simple things like turning on a ceiling fan in the room to circulate air could help with the temperature.
How is the tank temperature at night vs day when the lights are on ?
Light go off at 9:30 pm. When I left this morning at 4:30am temp was 78.50.4 temp drop is nothing to worry about, My only question would be what was the temperature in the morning before lights come back on.
It can take longer than a couple of hours for the tank temp to drop to its lowest point.
Overall I would think your tank temp is fine and not any issue with loss of coral.
Flow can not be overstated in the importance arena