A little more info would be helpful - how fast are you losing it? Has it been constant or is the rate increasing? Why all the component replacements, to fix the consumption problem? Is there a sweet smell or steam on the inside when running the heater? A pressure test is definitely in order! When you do this, make sure the engine compartment is clean and dry. Also make sure it is parked on a clean dry area so you can look for leaks on the ground. You may have a leaking heater core or possibly a head gasket that is slightly blown letting coolant into the combustion chamber or external leak. All head gasket leaks do not necessarily get coolant into the oil, so clean oil does not eliminate this possiblity. When doing the pressure test, visually inspect the head to block seal for any leaks. If it does leak down but you can not find the source, lift up the carpet to see if the heater core is draining into the floorpan. Lastly, if you still have a leak but can not find it, pull out your spark plugs and after letting the pressure sit for a good bit of time, using small mechanical fingers, stick a cotton ball into each cylinder to see if there is any liquid. I have never done this, but in theory, it would work.
I would recommend doing the pressure test with a cold engine initially (for safety and so minor leaks will stay in the fluid state, rather that steam) and repeat with a warm engine if required.
Not knowing you level of experience - be very careful working on hot pressurized coolant systems and dispose of any coolant properly, it is deadly to animals!
I will look for any replies should you need help.