As an American, I would use "to release the dogs on," in your first example, rather than "set their dogs on." When I am talking about "setting someone on someone else," I use "sic" more often than "set." "I'll sic my big brother on you!" The Oxford English Dictionary does say that "sic...on" is a North American phrase. It's definitely informal language, and very emotionally loaded.

The two phrases do have the same basic meaning. However, the first has more emphasis. "Once every week, I see my friend." In this sentence, the time and frequency are emphasized. "Every week I see my friend." In this sentence, the time and frequency are less important. Note: this only works when it is "once." "Twice every week, I see my friend," does not put any special emphasis on the frequency. Also note that we tend to prefer having one part of the equation be singular. "Once every three years" works, and "three times every year" works, but "twice every three years" sounds very awkward.

In English most words can be multiple parts of speech. While "open" can be a verb describing the action, it can also be an adjective describing the state of the door. "Watch" can be a verb, or it can be a noun, like Abigail has pointed out. You will see this with many English words. "Book" can be a physical object or the act of scheduling something. "I read a book." "I booked a flight." "Fly" can be an annoying insect or it can be the action of moving through the sky. Side note on slang: because so many words in English can be multiple parts of speech and because English doesn't have specific endings or forms for the different parts of speech, English slang will give words even more ways to behave. Even though "English" is a noun or an adjective usually, you may hear people using it as a verb "I can't English well today." (Meaning, I am having trouble speaking English today.) While that use is not technically correct, you may see native speakers doing so in informal contexts. I wouldn't recommend learning to do this yourself until you are really comfortable with English, but I wanted to give you warning that you might see others doing it.