Coolant will probably be the pink stuff but just put more of what ever is in there dont mix the blue and the pink, you tried parking the car somewhere dry and letting it warm up to temperature to see if you can identify a leak?

As you were suggesting about no coolant in oil or vise versa it could still be the head gasket say if it goes between a cylinder and a coolant channel, do you get any bubbling in the expansion tank? does the coolant smell of petrol ect?

The thermostat cracking is an issue on 1.8/2.0l engines never heard of it being an issue on the 1.6 but you can get them for a tenner on ebay worth a check.