the tabpro s is a windows tablet, there is no back button per se.
As for screenshot on portrait mode, since you don't have the hardware keyboard available, I see the following 3 options:
1. full featured keyboard or mouse connected via bluetooth (with mouse, the assumption is you have programmable buttons)
2. turn on enhanced virtual keyboard and see if the screen print button is on it, but you will likely get the virtual keyboard in the screenshot unless you do an alt-screenprint
3. some third party software that puts a screenshot macro in the task tray or something.