When topping off the coolant, leave the cap off the coolant tank and start the car and turn the HVAC control to hot. Let the car get up to full temp (190 degrees stock IIRC). This will help to ensure the coolant is flowing in and out of the heater core and using all the passages. I just replaced my thermostat housing / thermostat and haven't had an issue.
The oil to water heat exchanger (sandwich adapter) is between the oil filter and the block. From what I've seen... this basic design started on the first generation Lightnings and quickly migrated to the other SVT's and Ford cars as well over the years. The water reaches temp before the oil does and helps to get the oil warmer faster and helps to keep it at a safe consistent temperature.