Translation:Is the restaurant there?
If you replaced 那 (nà) with 哪 (nǎ), and removed the yes/no question particle 吗, it would mean "where is the restaurant?". Very similar character and pronunciation makes it confusing at first, I made the same mistake.
But 吗 should tip you off anyway since "where is the restaurant?" isn't a yes/no question.
Yes, always. If you want a more detailed answer you can use 呢 ('ne') : "饭馆在那儿呢？" meaning 'then' or 'so', i.e. : "Is the restaurant over there, then?" or "So, is the restaurant over there?". If you want to ask "where is the restaurant?", you would write: "饭馆在哪儿" (northern dialects) or "饭馆在哪里" (southern dialects). In this case, you would use: 哪儿/哪里 (where?) instead of 那儿/那里 (that, those).
Yes, "吗" always signifies a yes/no question. However, your "呢" example is wrong.
Here's how "呢" is used:
To soften a question or impart a quizzical tone:
- 饭馆在哪儿呢？Where is the restaurant, then? (You need a question word, "那儿" or "哪里", not "那儿" as in your example.)
To ask where something is (by implication):
- 饭馆呢？The restaurant, then?
To indicate an action in progress:
- 我（正）（在）吃饭呢。I'm in the middle of eating.
And here's another particle to consider:
What I think you might be looking for is the suggestion particle "吧":
- 饭馆在那儿（，是）吧。The restaurant is over there, I guess. (This is an implied question.)
"Is there a restaurant?" doesn't work. I don't know why "is the restaurant there?" should work instead. Enlighten me.
Are you asking whether "那儿" is interchangeable with itself in Chinese?
You should be able to translate it to whatever's appropriate in English. You can try reporting "over there" as a correct answer here if it's not accepted. I don't think it makes a lot of difference in this sentence.
@Canary - that's okay. Lot of things don't make sense till they are looked at in the right way. e.g. to a lot of people, your comment may not make sense. Particularly if they start wondering how can a sentence make anything anyway? We are supposed to make sentences. (Intended humour is apologised for in advance :-)