I wrote the FAQ that exists in the Raspberry Pi docs, and here, and was in something of a rush trying to tackle the common issues so I didn't go into much detail on this or power options.
You are correct, however, the Pi 2 has the second i2c available through the DSI ribbon cable to the LCD, so it can be used to initialise the display, control the backlight and read back touch coordinates.
The old Model A and Model B Pi's don't have this i2c bus, so you need to give over control of your main bus to the LCD (unfortunate if you want to use any other i2c devices) and connect SCL/SDA manually with the supplied jumper wires.
There are, as people mention, several ways of powering the display. By far the best is to use two 2A adapters and power the display/Pi individually. Yes, two mains adaptors is overkill for many setups, but it's also great for making sure enough power is available to HATs and peripherals without ever seeing a rainbow-square-of-low-voltage. I only use one adaptor myself with our split cable, and with an external hard disk connected, so I don't practise what I preach, but it's nonetheless the best configuration for making the most power available.
With the Pi 3, when the new official 2.5A adaptor is available then that would certainly be the best balance of power/portability. I haven't tested one with our split cable yet, but it's my personal favourite setup.
So to summarise, you can ( and I have tried all of these configurations ) power the:
- display from the Pi via jumpers
- Pi from the display via jumpers
- Pi and display from separate adapters
- Pi from the display via a USB to microUSB cable
- Pi and display with a single adaptor using a Y splitter cable
The two configurations I use regularly are the Y cable, and Pi/display connected via jumpers and powered by which ever port I bother to plug it into. I have jumpers soldered to the underside of the Pi to avoid blocking the GPIO header.
You could even power the display from a USB port on the Pi, but that would count toward the USB current limit (absolute max 1.2A) shared across all USB devices so I wouldn't ever bother.
