1) Yes, with some difficulty, changing one device to another would require trimming of the bias voltage and maybe a large heatsink may be needed.
2) Since gate v determines drain V, you would have to trim gate v to fit each diff device you plug in. Not the greenest choice for an amplifier, but possible. Gate threshold voltage varies from unit to unit, this is what you look at on the spec sheet.
So if you want the gate V to be 0 to 5 you might find only one small range of gate V works best, and different for each diff device.
3)MOSFETS are excellent switches, poor choices for amplifiers since their turn on is very abrupt(threshold voltage). Distortion can be high, depending on whatcha doin'.