1. Does the devices ID refer in DevIdx parameter in the above functions remains fixed in every activation of the software?

Not necessary. In every new binding of the DLL there is new enumeration over the USB devices, Therefore the ID might change according to the system state. If you need to refer to a fixed device independent from its current device index you should check the devices serial number. The devices serial number is returned by GetDeviceSerial function.
2. Does the call to SetUsbDevice function is obligatory in order to access the device?

Yes, the user must call the SetUsbDevice in the beginning of his application in order to use the above functions.
3. Can I dynamically add USB devices while using the above control functions?

When a new USB Light Module device is connected the SetUsbDevice must be called in order to enumerate and control the new device.
4. Can I use multiple USB signal towers on one PC?

Basically there is no problem in doing so. But as mentioned above there may be some problems with the power supply because some of the USB ports of the PC may share the same power supply on the mainboard. In this case use an external USB hub. Please make sure that the power supply is able to provide 500mA supply current for each connected module.
5. How important is the cable?

Because of the current consumption of the USB device of up to 450mA it is important to use a cable of good quality. Cables of USB 2.0 standard should be used although the USB device operates on USB 1.1 standard. If you need to use a longer cable than the one shipped with USB element make sure that it has a wire diameter of the power lines of at least 24AWG or better.
6. The USB module is recognized by the computer but fails if I turn on the first light!

Some laptop computers do not provide enough power on the USB port for the USB element to work properly. This problem may occur also on some standard PCs. In this case you should use a USB hub with an external power supply to connect the USB devices to the computer.
7. The USB module is still not working correctly!

Maybe the problem is with your motherboard chipset driver for USB.
Make sure you have the latest driver for your motherboard installed.