LCD leaking power through I2C cables
You don't need level translation for I2C as it operates with pull-down only, you just need to set your pull up resistor to the 3.3V.

In general, unless otherwise specified by the datasheet, I2C pin cannot be higher than VCC, and if you power down your lcd, they will be. The reason is the embedded esd protection diode that will feed your lcd through the I2C line. 

It's not easy to power down a device on a bus like this,you will need some specialized I2C chip for this porpose. 

You rather check if the lcd has a sleep mode as most do.