Hello, this is not possible. The buttons cannot be assigned individually. Either TV or AV or set-top box.
You always have to switch to the corresponding device.
Yes, that should work. If in doubt, you could ask the manufacturer One for All:
https://www.oneforall.de/universal-fernbedienungen/urc-7140-essence-4?_ga=2.256812814.400066606.1621069351-1519186906.1621069351#/step-1
Unfortunately, I do not know whether several units can be switched on and off at the same time. For the volume, I have set the volume control of all units to the same button on the remote control. As before, the volume is only controlled on the unit that is currently activated. Since the units are physically connected in series (with cables), the control on one of these units affects the entire chain. If desired, after switching on, the unit that is controlled and switched the most can be selected as the active unit. To switch back and forth between the units, only one button has to be briefly pressed two to five times. That is practically no effort. For me personally, it is the best remote control I have had so far and at a sensational price.