Welcome to the forum. Mileage is determined by driving style, conditions and cycles. This is explained in your owners manual. If you don't have an owners manual, you can download one for free.
Who's "they"? Why didn't you ask them what piece they were talking about? Check the sticker under the hood to find out what type of refrigerant it takes.
Welcome to the forum. All I can suggest is to be aware of the weight limitations on the roof. I believe it's like 300 lbs. Realtruck.com has a few choices of bolt on roof racks.
Agreed. Could be cheap bulbs, an issue with CANBUS, etc. Put the regular bulbs back in and see if that corrects the issue, if so, the LED bulbs you got are not compatible. Welcome to the forum.