I removed the intake manifold/throttle body assy yesterday and found about 0.5L rat crap on top of the engine, that would explain the funny burning smell every time the engine got hot.
I also found that the 'ceramic ring' I saw was indeed the spark plug obscured by my crappy endoscope.

I did a compression test and got the following values:

Cylinder : First test : Second test with oil added
1 135 kPa 145 kPa
2 145 kPa 175 kPa
3 130 kPa 180 kPa
4 150 kPa 170 kPa
5 0 kPa 0kPa
6 130 kPa 180 kPa

WIS states that compression should be between 120 and 140 kPa so the values look good, cylinder 5 being the exception. I could not get a reading at all, the needle on the gauge wouldn't even move. I double checked the connections and proceeded to cylinder 6, getting a reading. I returned to cylinder 5 and still got nothing.
I believe that if the rings are worn I would see some pressure especially with the oil added. If the head gasket is blown I should still see some movement in the needle.
I suspect the exhaust valve is damaged or stuck open.

Any advice?