Yes. For questions you put the verb first. Most languages do understand a person who uses vocal inflection to change a statement into a question, but it is not standard. Spanish (and I believe Portugese though I am not that far along) does use it as a standard form under certain circumstances. One of the reasons for this may be that they often drop subject pronouns anyway, but it may be why the begin written questions with ¿.
No. Doch is essentially to the contrary, yes. It is always said in response to a negative statement. So it would be appropriate if a Grandfather said Ich bin kein Großvater. Doch is why it's even harder to raise a child in German. After they go through the no stage, they go through the doch stage where you hear doch whenever you say no.
Well actually this is a rather common question that I have heard men asking their daughters or daughters-in-law whether they have delivered their baby yet. But otherwise, considering there are so many people who don't know their fathers, there are probably many men out there with grandchildren they know nothing about. It may be sad, but it's definitely true.
That's an interesting question. Romance languages use that sentence syntax quite regularly. In early lessons Duo tends to discourage it because it is much less common in English, but later they essentially allow you to translate whatever syntax is there. I realize that I have no feeling as to whether or when German may use it. I learned German in Germany by immersion, so I assume that they either never use it, or they use it as we do. If they used it differently I would have noticed.
If they never use, then that syntax would be a valid alternative translation. If, however, they use it like we do to indicate surprise, then it should only be translated that way when it is used in German. No English class teaches it as an alternative, although it is a not uncommon spoken practice, so it's somewhat hard to categorize. But on the theory that it is a good practice to keep these answer databases as lean as possible, that alternative translation really has nothing to do with the German and it something you already know when to do in English, so it doesn't really contribute anything to this German course.
Opa is the affectionate nickname for grandfather. Personally I don't think we have quite the same style of nickname for grandparents, which may be why so many of my friends who had any sort of non-English heritage generally referred to their grandparents by words from those other languages. But to use the only really common options we have, opa is grandpa or granddad.