Answer to Question 1
You should never flush through refrigeration components that act as natural traps, such as liquid receivers, accumulators, or compressors. Do not flush through the metering device; the restriction in the metering device can trap small particles and stop it up.
Answer to Question 2
The refrigerant oil acts like a scavenger, absorbing contaminants as it circulates through the system. The compressor can hold large quantities of contaminants suspended in the oil and settled at the bottom of the crankcase. The fastest way to clean a contaminated system is to keep changing the oil until it is clean.