why external of 5V whouldn't work ?)
Because that value of 7 - 12V is the voltage to the power jack. If you want to power it off 5V then feed it direct to the +5V pin on the arduino.
Just add up all the currents and that is the minimum you need. If you don't know the current of each device then you can't work it out. You will have to measure it.
In order to get 3.6V and 2.2V i will use some kind of resistors or another 2 different power supplies
No you can not use resistors. You can use voltage regulators to get these lower voltages from the 5V line.