Ignition condition vs MPG

just wondering how conditions of s.plugs&i.coils affect the MPG. I was getting low MPGs around 40-45 since the summer. And eventually the car started to misfire severely. I thought all other common problems first, and then when I removed plugs and coils, which were wet and coils had some rust. Cleaned and dried up all those, also inside the engine tube was kinda oily carbon gunk. So surprised to see that, but cleaned all the gunk, and installed the things very protectively with anti-seize grease and boot grease.
The misfiring is gone, after 16miles drive it got 48.5 mpg.
So I ve following questions also:
Does any of cylinder misfire occasionally during a ride due to bad condition, and this worsens the Mpg?
Second, how dirt or gas leak thru ignition coil side? Might it flooded engine?

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almost 250k, will get deep cleaning soon. Own it for 2 years