The apparent low oil pressure is most concerning. Try fitting a temporary (or permanent!) real oil pressure gauge to give you a better idea of the actual pressure under various conditions. The warning lamp sensor, circuitry, or wiring might be sending erroneous signals.
Oil pump wear is not unheard of, probably the most probable suspect.
Running higher viscosity oil is worth a try, but not a complete solution to the underlying problem. Maybe also try overfilling (1L?).
High oil consumption would have us take a look at the oil control rings first, then maybe valve seals or turbo. How bad is the blowby? Have you checked the compression? Has the engine been overheated (could permanently relax the piston rings)? Has the engine repeatedly been run under high load before reaching operating temperature?
The fact that there are no external leaks would make me want to save this one for sure!
