I have a 2015 Sierra
1500 SLT with the 6.2 liter engine and 8-speed box. Also the max trailer tow package. There are 2 modes to the hill descent control. The more aggressive mode is invoked when you use cruise control; the transmission will start downshifting when you exceed the set speed by 5 mph and will continue to downshift if the vehicle keeps accelerating spinning the engine up to about 4200 rpm. The second mode is less aggressive and kicks in based on some combination of factors that I can't figure out. The nice thing is that when cruise control downshifting happens, applying the service brakes does not trigger an upshift. You can use manual control as well, without fear of over revving. The shift command will be refused if it would cause an overrun.
Regarding the use of "auto" on your 4wd setting, I don't see why that would provide any benefit. In fact, if you're in a low-traction situation (rain, snow), you should be careful about using a lot of engine braking. Use the service brakes instead and slow down a lot. Remember that most newer US-branded trucks with integrated trailer brake controllers include ABS and D.SC. Functions.
I've descended up to 10% grades without incident, using the service brakes as needed and never got them hot enough to smell. That's pulling an FC27.