I've always removed the reservoir for cleaning. Noticed the same thing when draining the radiator; that it wouldn't empty. The system works as I've noticed the level dropping and raising with engine temperature. My guess is that the system finds it easier to suck in air from the opened drain valve than suck in water from the reservoir. Since water is denser than air...
You can have oil tested for coolant content. You wouldn't necessarily notice any changes in the oil unless the gasket was just completely blown between a coolant and oil passage. But that's not a usual failure point for these vehicles. (although a crack in the head can lead to the same dumping coolant into oil) If the coolant passages are dripping into the cylinder then most of it is being burned-off instead of ending up in the oil. When you stop the engine the liquid may be allowed to sit long enough to get past the rings dependent upon their condition.