Hi TF12
Ammonia loves water and over a long period of time loses some of it's volatility and will remain as a liquid even at atmospheric pressure. This is referred to as dead ammonia and it is in all old systems to varying degrees.
Ammonia does not need to be recovered, so when you need to work on a part of a system all you need to do is pump down and then the remaining ammonia can be blown down into water. If you are in a situation where there are no people this could be blown to atmosphere as ammonia is a naturally found gas and causes no harm to the environment. If the system cannot be run there are recovery systems but to my knowledge these have been purpose built by industrial refrigeration companies, there are probably commercially available units out there.
I am not sure what you mean by draining by gravity, are you referring to dead ammonia which will sit in low points after the system is pumped down and is usually drained out of the oil drains.
If you lose oil out of the compressor or separator it will not be returned to the compressor as it does not mix with ammonia and will settle out in vessels and heat exchangers and will have to be drained, this is the reason for oil drain valves on numerous parts of the system.
You have got the first rule right, always treat ammonia with respect.
There are many ammonia guys on this forum who may add to what I have said.
Paul