C1E is part of ACPI, it controls the processors power states. On being C0, C1 or C1e is the halted state, and c3 is the sleep state.
CoolnQuiet reduces CPU speed (via multiplier) and voltage while its idle.
Is it safe to leave it running 24/7 - sure. ; Just as safe as turning it on and off a few times a day. As long as you have the bios set properly to shut down in the event of an overheat or fan failure that is.