Here's something to remember:
Don't confuse "critical" with "non-magnetic".
Almost completely, across the boards, the steel we use in knife making will go non-magnetic around 1413 degrees - give or take a few degrees.
What is changing at that temp is the nuetrons and protons in the IRON. It's the iron that is going non-magnetic.
The "critical" temps, for the specific steels, is dependant upon the other alloys.
Using magnetism is only an indicator that you are approaching critical temp.
I'd listen to Rob on his experience!
With only a coal forge, and the high austenizing temp of D2, get another steel.