I dont think I would eat that lettuce. Those white spots sound iffy but the top and brown is just wilted and wouldn't hurt you. I would go buy some new lettuce if I were you
It is best to chop the garlic in fine pieces but you have to watch it in the pan so it won't burn. In my opinion it is easiest to buy a jar of minced garlic and on the jar it tells you how many teaspoons or tablespoons=so much fresh garlic