Hey mate, the stalling issue sounds like one of two things so far:
- Vacuum leak (check all your hoses and piping)
- BOV (I presume it's a vent-to-atmosphere one?)
If you don't have an aftermarket ECU (be it a full standalone like Power FC or a piggyback or a re-mapped stocked ECU) then I'm fairly certain that it would eliminate your running rich problem, hesitation problem, as well as your stalling issue (provided it's not a vacuum leak and is BOV related).
As for oil on the plug, that doesn't sound very good but have you got a compression test done?