Placement of sensors, Gyro + accelorometer
From a control system perspective it really does not matter where you place your sensors.  The most important thing is that you know where they actually are!!! I mean this with regards to knowing where the sensor is relative to the centre of the rotary wings.  As long as you know the offsets you can apply translations to calculate what your vehicle is doing.

From an electrical and performance perspective away from motor wiring and h bridges, away from vibrations sources and suitably mounted and damped.  The dampening is important for the accelerometers to ensure that they do hit mechanical rails from high frequency vibration (do the math to work out the maximum displacement your accelerometer can with stand at various frequencies and you will understand this point).  Similar for, but not as important for the gyros.