Unpowered devices on I2C/SPI bus
1. Do they really mean Vdd+0.3V even if Vdd is 0V?

Yes.  Second guessing datasheets is a really bad idea.

The problem is that most chips will have protection diodes from inputs to the positive supply.  These are to keep the device from getting hurt from small static discharges during handling.  When the device is powered down, these diodes will either clamp the line externally to one diode drop above ground, or partially power up the device so that it's power voltage is one diode drop below the signal voltage.

Unless you find a IIC device specifically designed to be powered down while not in use, there is no good way around this.  IIC is bi-directional, so simply buffering the lines into the device won't work.

The better solution is to find devices that draw very little quiescient current.  A IIC device that is not talking on the IIC shouldn't need to draw much current.  Some devices may have a deliberate powerdown mode for that reason, which probably keeps the IIC front end powered up but with minimal current draw.