1) What harm could blowing out the silt with compressed air do?
Probably none, unless the outward pressure were to cause the porous water layer to compress and seal up. But the bigger question is HOW you would do it. Unless the static water level in the well is up to the top of the pipe, so you can cause the water and silt to overflow it, all you will be doing is roiling up the silt and it will resettle to the bottom.
3) What pressure and volume and pipe size do I need to use (I assume I would remove the pump and use the same pipe with an extra joint?)?
If the well is full of water, then you would need about 270 psi, just to get the air to the bottom of the well then you would need additional pressure to disturb the silt.