Wondering if glow plugs get triggered below 75° Celsius on M57 diesel?

[ninja_zx11]

Hello friends, Was reading BMW technical training material and some european diesel forums about the normal running temperature of M57 diesel engine on 2009 X5D.
In the training material,it says glow plugs works untill the temp warms up to 75° Celcius.My 2009 X5d's engine temp sometimes on highway with constant speed goes below to 73° C from 78~80 in the city stop and go traffic.I think my thermostat is bad and i am going to change it.

But few questions are arising in my mind:
1.Do glow plugs start working again if temp goes below 75 after the engine has reached temp over 75 once?
2.Will running engine below 75°C effect active or passive DPF regen?
3.I showed it to the dealer and they say that if there is problem with the thermostat, then it will trigger some cooling system error code.But i read on some diesel forums that M57 should run at around 87~88°C.So as my engine's temp is going below 75,why its not triggering any engine code?
Looking forward for your suggestions.thanks