I'll try to answer you questions.
1. The Smart RTH is calculated as the power required to get from the current aircraft position, climb to the Failsafe Height (user set), return to the HP and land. It is indicated by the Yellow Dot with the H on the power line. It will move to the right the further you are away from the HP and to the left as you return to the HP.
2. The Low Battery Warning and Critical Battery Warning are set my the user in the Battery panel. They default to 30% and 10% respectively: the lowest you want to set these is 25% and 10%. When the LBW is triggered you get a red flashing status panel and a verbal warning. The battery percentage also turns red. You need to plan to return to your landing site when you get this warning. If you are a long way from the HP the Smart RTH can activate before you get the warning. When the CBW triggers you will get the announcement and warning that the aircraft will land in 10 secs. You can cancel this if you are about to land but you don't have much flying time left.
3. I am not sure I fully understand your question. When the Smart RTH activates you will see the red circle and white cross bouncing on the left side. You can cancel the RTH by tapping this button. Alternatively, you can cancel any RTH by pressing the RC RTH button (properly the easiest way) or by flicking the RC PAF switch to ATTI and back to P-GPS.
I hope this helps your understanding.