Japanese here. I find it fine to say ありがとう for the first two, although どうも is more common. Not saying anything is perfectly acceptable. You can also nod, which is very common.
When you leave the restaurant, it is common to say ごちそうさまで～す or ごちそうさまでした. If you are female, ごちそうさまでした would be more common. It is perfectly ok to leave without saying anything. Nodding is very common as well.
It is true that in Kansai area (particularly in Osaka) and also in rural areas people tend to say something, compared to Tokyo for example.
