If mains fails and my device is working in battery mode I would like to disable Ethernet, USB, HDMI, turn off the LCD backlight etc. Is it possible to disable these modules? My device battery is discharging very quickly, and I am looking for anyway to save power. This is first time working in power management.
As is the Raspberry Pi doesn't support a Deep Sleep mode as you describe it. Most of these modules can all be disabled, but the savings are minimal and they will all require a restart to toggle on or off.
Turning down the brightness will surely help (presuming it's powered from the RPi), but how and if you can do that will depend entirely on the screen you're using. Check its documentation for information on that.
As is, the RPi uses remarkably little power. The SOC (CPU, RAM, GPU, etc) is going to be eating the vast majority of your power. If you need your device to last longer on a battery, the best answer is to simply provided it with a bigger battery.