Reading your posts, I’d like to make some annotations.
Most electronic components can take at least up to 80°C, but at the cost of reduced lifetime. So, the cooler they run, the longer they last.
Components who have to control substantial currents like stepper drivers, MOSFETs for fans, heaters and the like, or voltage regulators tend to get hotter than other parts on the PCB. Without thorough tests and measurements, it is quite difficult to determine cooling requirements in advance.
Air flow is essential. If the heat of said components is not moved out of the way, it piles up and results in dangerous hot spots.
In contrast to common belief, most of the heat is dissipated through the back side of the Duets - mounting them flat down horizontally can be less than optimal.
If you intend to put your Duet into a pre-heated environment (say: a heated chamber), you must calculate this as an offset to the cooling requirements of the affected components. Even summer temperatures of more than 30°C may put a passively cooled board at risk.
At an ambient temperature of 60°C, the use of a fan is mandatory. In addition, you also should feed it with fresh air from the outside (to take 5. Into account).
If you are in the design process, you are good advised to provide a fan, anyway - if it turns out to not be required, you can turn it off. If you encounter hot spots, if you need to ramp-up the current for one of the steppers, or if the sun shines bright… well, you’ll be lucky to have a fan ready.