Options if autobody shop states part was faulty (Spark Plug)?

Car X installed new spark plugs in my 2007 Ford Explorer 10 months ago. Just recently, Explorer began to run rough. Got a diagnostic at Firestone and the result they came up with was Spark plugh in #3 was 'broken'. This should fall under the auto body shop's warrenty. I will be taking the Explorer into the autobody shop that installed the plugs last June tomorrow morning for their inspection. What are my options if they say it's a faulty part (I really don't expect them to say it was a bad installation) ?

Why is my car doing this?

I own a 2002 ford explorer xlt with 164k on it and when I drive on the highway for more than about 15 minutes it will not go past 3000 rpm and will not downshift. Also, when I try to go past 3k it will clunk and grind, even when off the highway on regular roads it will clunk (only after going on the highway). Oil is fine, can't access trans oil as it needs a special machine and has no dipstick. Any thoughts?

Hear me out on this I'm considering getting a 2004 Ford Explorer xlt or a 2005 jetta 2.5?

The Jetta gets 22 city 30 Highway. Tank capacity 14.5 gallons which means 319 miles city 435 highway while on the other hand the explorer gets 16 city and 21 highway with a fuel tank capacity of 22.5 gallons which would mean city it'll get 360 city 472 highway wouldn't the explorer get the better gas mileage? But yes it is more expensive to fill Jetta would cost around 34 dollars and the explorer around 54 dollars

I turned 16 and I need a car under $15,000 that I can work on myself?

My dad and I have been looking for a "project car" also everyday car that i can customize and put an exhaust and air filter.etc. I have so far looked at a 2003 toyota sequoia and a ford explorer sport trec. I'm still looking for a car with "okay" MPG and also reliable and safe.
any suggestions help