It's not overfilled. In fact my engine has a substantial oil leak, particularly when I've just parked it (since the oil is warmed up the viscosity allows it to run through wherever the leak is quicker). It drips directly below where the gearbox meets the engine... but that could be gravity bringing the drip point over.
Doubt the oil filter is blocked, I cleaned it out a couple of weeks ago.
No idea about the other oil return runs... presumably they're inside the engine, so I can't check.
The oil in the breather pipe is creamy, however the oil in the engine itself is fine (I can tell from both my leak and the dipstick being black as night!) Occasionally when I look at the oil filler cap it has a layer of cream on it too.
Needs a good machining/rebuild I reckon!