Clean off the dipstick and under the oil filler cap thoroughly and see if the foamy stuff appears or returns. I have this on my 95-850NA whenever the temperature is fluctuating daily (winter/spring) and when the car is used for frequent short trips. It goes away when weather is warm and/or car is driven longer. Other posts have reported similar.

Also check to see if a hose or clamp is leaking or there is a small crack in coolant reservoir or cap. Over the years I have had about 3 small leaks which will result in draining of about half of the coolant from reservoir over a few weeks. In these cases the system almost but not quite holds pressure and the leak can be hard to find. Are you sure the coolant smell is related to the heater core and not blown in from under the hood?

Your oil usage seems small. I am adding about 1 qt per 2000 miles (122000 miles). Much less than a half quart can be hard to tell unless the car is always at a level spot or at least the same spot and also has had time for most all of the oil to fall down into the pan. Any visible oil leaks?