Since most of the oil coming out of the breather hose would go into the front throttle body, maybe check to see if the plugs look different front to rear? Surely that kind of oil consumption would leave evidence on the plugs..
I'm not familiar enough with the 990, but on my 950 oil can go almost directly from the breather hose into the front carb inlet via gravity and crank case blowby air flow.. anything getting onto the filter is spray or what has run down the top and side of the box.
All blowby gases must go through small holes drilled radially in the left counterweight, which should have significant centrifugal effect. If that balance shaft seal is bad, however, you lose this system built into the breather to "sling" the oil out of the airflow.