Need help to fix the problem of borewell motor which is stuck in mud


We have a borewell with a motor at 500ft depth. During rainy season, there was a little seepage of muddy water and the motor stopped functioning. When we checked with a borewell person he told that borewell motor is stuck with mud and cannot be pulled out. So we had a pipe blasting and pulled out the remaining pipe and thread. WE were told that after this we annot use the motor. We scanned the borewell using a camera and fund that there is another source at 180 ft and can use that by installing a new motor. Please help me with the following points.

1) Is it safe to use the water by installing the new motor as the old motor is still in the well?

2) What is the risk of facing a similar incident again? How can we prevent this? 

3) What is the opinion on compressible pump? What will be the pressure of water? Will we be getting only muddy water now? 

4) Is there any problem if septic tank is located nearby?