I2C capacitance problem
when a switch is in the 'off' state, will the switch itself contribute to the bus capacitance?

Obviously yes.  The switch isn't going to have 0 capacitance.  How much capacitance it has is, of course, is something you look up in the datasheet.

Keep in mind that some IIC slaves can drive the clock line low to perform clock stretching.  This won't work if the switches aren't bi-directional.  If you know for sure that your slaves won't try to drive the clock line, then you can use uni-directional switches (buffers).

Putting 400 anything on a IIC bus line is a bad idea.  Whether these connections go to the inputs of analog switches or the end devices themselves, there will be considerable capacitance.  That could be overcome by running the IIC bus slowly.  However, you need to check how much the leakage currents add up.  There is a hard limit there.

All around, you should step back and think how to solve this problem differently at a higher level.  This just isn't the right way.