My responses/opinions:

1. Idle temperatures could be anywhere between 30 and 60. Why such a big variance? Because Windows 10 run a *lot* of maintenance (and spyware) activities in the background. "Idle" is just not what it used to be. I would hope that, with the quality of your thermal solution, your idle temperature is mostly in the lower half of this range.
2. In Desktop systems, your cooling solution is, by design, supposed to be maintaining temperatures at or below the processor's "Tcontrol" temperature (see my discussion of Tcontrol (etc.) here: Re: I3 8350k temp).
3. Any temperatures that are above your processor's Tcontrol temperature that are sustained for long periods of time could potentially cause thermal degradation in the silicon and this could shorten the lifetime of your processor. Having short duration spikes in temperature, even those that go close to your processor's Tjmax temperature, do not cause such thermal degradation.
4. My definition of "Optimal" is maintaining temperatures at or below your processor's Tcontrol temperature.
5. Downclocking is safe if it is done properly - and yes, it can lower average temperatures. Whether it causes any lag in your games depends upon the game and its requirements and how much you downclock it.
6. Throttling starts at the Tjmax temperature. See link to my discussion above for more information.

Hope this helps,

...S