Modern DeVilbiss (and other) guns are either HVLP or "Compliant" guns or conventional, pre-HVLP guns. For a description of HVLP vs. Compliant, see here:
The absolute best way to easily enable spraying the underside of something that can't be re-positioned (upside down painting) is to use a remote cup arrangement where the gun itself is attached to a completely separate paint cup with air and paint hoses. Another benefit of such an arrangement is your gun hand doesn't have to carry the weight of paint in the cup. The gun in such an arrangement needs to be set up for pressure feed with the appropriate fluid tip and air cap. Here's an inexpensive setup:
Both DeVilbiss and Sharp currently have such arrangements but they would be a LOT more expensive. My own remote cup arrangement is a DeVilbiss one (conventional) that is over 30 years old. For me at least, the present cost of the same arrangement from DeVilbiss would be prohibitive - likely in the $700 plus range!
I've painted in the 50's but prefer it to be 60 deg F or above, but below 90. Sweet spot is between 70 and the low 80's. One thing that is important if you are painting outside this time of year is make sure the temperature stays well above the dew point over night. Otherwise dew could destroy the appearance. Freshly painted horizontal surfaces are especially susceptible to damage by dew. You should be able to find some good days this month for painting outside if you can move the project inside before evening.