Hi, sounds frustrating! If the system isn't pressurising and your absolutely sure there is no signs of oil in the water, i've had head gaskets leak between oil galleries and water galleries before, the old Rover 200, K-series engine used to do that before Rover modified the gasket, also had heater exchangers internally leak as well, but not sure if the 159 is fitted with one. I'd leave the head alone for now. You could try and blank off the matrix, we used to use a coupling in the trade that basically meant you dropped off the in and out of the matrix and joined them together. I know you've done your swirl flaps can you maybe pass a camera ( or something ) inside the manifold to make sure its not leaking in there? Also my 159 takes a while for the level to settle after being disturbed usually about a week of small top ups before the level remains stable, but once there it doesn't move.
Hope you find it soon!