Cessna 172: at which RPM should the oil pressure reach the green arc?
Your answer will be elusive.  The problem is that oil pressure is a function of the oil, oil temperature, time, engine pressurized oil draw (the "wear" of the engine), and RPM.  

I am confident a model could be created using the above parameters, and possibly one or two more.  However, you are probably not interested.

I have about 8000 hours of instruction given in C172M, which is a different engine.  With 100 weight oil, at 20C or higher, I teach that oil must be moving up within 5 seconds, and with 1000 RPM it must be approaching or in the green within 10 seconds or there is an engine shutdown.  

In contrast, with 80W oil, at 32F (we require preheat if engine temp is below 32F and the aircraft is at home base), I teach that the oil pressure must be moving within 5 seconds and in the green within 15 seconds.

These times are on our customized checklist, and do not address cold weather ops although they are quite common for us.

We will not start taxi until the oil is in the green arc. We always look for movement of the oil gauge within seconds after the engine start. We bring the engine above 500 RPM, but not necessarily to 1000 RPM waiting for the oil to rise up.  A factor in not going to 1000 RPM immediately is that the engine wear without full flow on a journal bearing is the square of the surface velocity. On the other hand, the pump volume is nearly linear with respect to the rotational velocity.

Also, note that we use straight weight oil 90% of the time.  Multivis oil will behave differently.

To address your two questions, I propose you use the following:
-1000 RPM for oil check all the time

500 RPM idle typically yields 30 to 35 PSI