1- no, more likely to hurt mpg and/or driveability. You are not an engineer, and the intake was designed by an engineer with sophisticated computer models and tested.
2- no, K&N filter is expensive. An over oiled filter can restrict air flow. So there is a risk you might not handle the filter properly in the cleaning and reoiling process. Cheaper to replace your air filter as a DIY weekend chore.
3- marketing hype, they take your money and you “feel” the difference but it is a placebo effect. After spending the bucks you imagine you get some benefit. Dyno testing shows no difference.
4- very expensive, not likely to recoup your $$$. The main benefit is perhaps a nicer exhaust note, music to your ears. Higher mpg is 0 to minimal. Many drivers like the exhaust sound leading to more push on the gas petal to enjoy the sound - there goes your mpg!
Keep you tire pressures at recommended levels, lighten the load in the bed, drive gently, reduce your cruising speed on the highway. Slowing from 75 mph to 60 will give your much more mpg than 1-4 above.