1. As the spark plugs wear over time they loose there integrity and the ability to burn all the fuel in the cylinders. This is why the problem is not showing up till now.
2. You are correct in your thinking almost. The engine can not control how much oxygen it takes in. Please do not think I am saying you do not know what you are saying. Basic physics tells us the colder the object the denser it is. The colder the air the smaller the oxygen is, thus the more oxygen you get for the same amount of volume of hot air "same for the fuel". If you drive the car the exact same way hot or cold out side, you will use more fuel in the colder days then the hotter days.
3. On colder days the weaker spark will be able to burn more of the air and fuel due to the added amount of air and fuel the engine has taken in. On hotter days there is less air and fuel taken in so the spark has to burn stronger to be able to create a better combustion in the cylinders.
4. If the spark plugs are burning the fuel in all cylinders the same then no the engine will not miss, shake, or set a trouble code in the pcm. The pcm only detects if a cylinder is off balance from the rest of the cylinders.
I hope you understand I am not a physicists, I am a auto technician. I hope I have made it easy for you to understand and I am sorry if I am not succeeding. Have the spark plugs replaced and see what happens with your next two tanks of fuel. Make sure to use only factor recommended spark plugs. Most tune up places do not use factory replacement plugs," they use the cheapest thing that will make your engine run". At least that is what most do here in the states. Second of all, using the cruse control will use more fuel then if you where to control your speed. Cruise control does not hold the speed at a set speed, It will accelerate to get you up to speed then will back off till you drop to a certain point then accelerate again to bring you back up to speed. If you control the speed your foot will hold the throttle at a more constant speed and use less fuel.