"Dobrý večer" ("good evening") is used as a greeting during the evening and night parts of the day. "Dobrou noc" ("good night") is used as a wish before going to sleep, so no, these are not interchangeable.
No. "Good night" means "dobrou noc". We only say that before we go to sleep. It does not mean "dobrý večer", you wouldn't even use "dobrou noc" to say hi to someone during night hours.
This does not make much sense to put into one sentence. Good evening is said when you are meeting someone, thank you when you are grateful. Seldom used together, usually separately.
These phrases are directly adopted from the original DL courses. The point is that there is too few of them so they are combined to have more exercises. This was not invented by the creators of this course.