Hi jp11,
There is an o-ring in the plastic clip that the rod with the flaps slides through This then sits in the top of the engine. I think after a while this o-ring wears a little and some oil weeps out. I have a similar thing but didn't think it was enough to worry about.
In answer to your second point, from what I saw there are two inlets per cylinder, one with the flaps and one without. At lower revs the flaps are open to allow as much air as possible in to the cylinder to give a good burn and reduce emissions. At high revs the flaps are shut as enough air flows into the cylinder. A description of this is given in the PDF document I have attached which was given earlier in the thread.
In my case the trouble was, when the link broke the flaps were left open meaning performance at low revs is ok, but at high revs there was too much air, hence the performance was reduced. The error that was picked up by the ECU in my case was Mass Air Flow Sensor (MAF) sensing too much air.
Therefore if you I had locked it open, I would have had reduced performance at high revs as there would be too much air flow. I am not sure what would have happened if I locked it shut...
For reference, on my car, open is with the joint pushed towards the engine, closed is with the joint pulled away from the engine.
I have now covered 300 miles and the mod seems to be holding up. Let's hope it continues!
At least we don't have BMWs which apparently have a habit of sucking these flaps into the engine!
I hope this helps and I hope you can get a fix sorted
Cheers,
Ed