I guess according to several of the posters, EVERY truck I have ever owned had something wrong with it as my temps have always increased when pulling a load through the mountains.
My expectation is that the temp would increase. And in the scenario you mention, I could see it going to the very high end. I had the same thing happen to me back when I had my 01 CTD. I was pulling a 10000# 5er, pulling it on Hwy 50 between Montrose and Gunnison, got caught in construction and couldn't keep speed up, the temp gauge reached the top of normal before I could gain some speed/RPMs and get it to cool down. The truck was brand new, so it was not a radiator/cooling problem. Just a matter of circumstances.
To answer the cooling down period, there are times where we stop at the top of a pass (there is almost always a place to stop pull over) and check my load/use the bathroom/walk the dogs. When I do this, I always make sure I let the truck idle for 3-5 minutes.
And, a side note, it is good to see ib post. Hope you are doing well.