While cleaning the MAF sensor with carb cleaner is not a good idea as it can damage it - always use electronic parts cleaner spray(auto parts store sells it), in this case if it did damage it you would have a code for the MAF sensor and more than likely a lean code such as P0171 or P0174. Also the Oxygen sensor would set its own code as well.
The grey on the spark plug indicates that it has good fuel/air ratio and burning the fuel good.
Just to eliminate a possible sticking valve or intake leak, I would suggest doing a cylinder compression test to see if #5 cylinder is 10% or more lower than the other cylinders.
If ok, I would next get a noid light and test for continous signal from the pcm to the injector. I have seen where the injector fires but erratically causing the misfire code.
Noid lights are about $10 at a parts store:
This simple plugs into the injector harness in place of the injector. crank engine and observe for steady pulsing light. If not present, then either a loose wire at terminal, open/short in wiring, or faulty pcm.
You can eliminate loose wiring at terminal by doing a simple wiggle test. Wiggle wiring harness and see if misfire smooths out.
If it doesnt, then physically inspect harness for nicks, cuts, frayed wiring insulation, or exposed wiring.
If ok, then the pcm needs to be tested with a factory level scan tool for proper signals going out to both the injector and the coil