The symptoms don't match the coolant loss.If I drive 20 minutes, I have to add coolant after it cools down. The coolant level is always low after a drive longer than 10 minutes.
I just bought an air compressor and I have a leakdown tester on order, so I should be able next weekend to get some air in that cylinder and see.
If you are loose coolant from a location other than the coolant tank while the engine is running, I seriously doubt it's going into a cylinder. The pressure in the cylinder is many 1,000s of Psi. This always allows combustion products into the coolant. Not coolant into the cylinder, as this only happens after shutdown.
More tests are needed I think.