Sounds nasty! I suggest the following:
1. Do a compression test using the starter motor. Do it hot and cold, wet and dry and record all of the observations and post the results here.
2. The only way oil can be getting through the inlet tract, is when it is pushed there by the piston with the valve open; therefore; oil must be getting past the piston rings, or through a hole in the piston. It can escape past the valve if the valve is burnt; or if the valve clearance is too tight; or if the inlet valve is sticking open; and without knowing the valve duration, possibly when the piston starts to come up on the compression stroke.
3. Oil can also get into the cylinder past the valve stem, but this would be unlikely to cause misfiring and it wouldn't be enough to blow up the inlet tract, it would probably be burnt before then.
4. I am guessing that No. 7 has serious piston/ring/cylinder wall wear.
I am also alarmed at the colour of the oil; it should never be allowed to get that black. I change the oil on mine every 10 000km and the darkest it has ever become was a light tan. Dirty oil does not help piston rings, or the camshaft.
Hopefully I am wrong and you can get out of this cheaply,
Cheers Charlie