Yes. When people talk about open war, I think they're being silly, but I think the threat of an armed populace engaging in small-scale warfare against a sufficiently unjust set of government officials is a non-trivial benefit.
Maybe. As above, I think an armed populace is capable of having a non-trivial impact, but I greatly doubt it would be capable of pulling an actual coup, at least not without having gained substantial military influence.
Frankly, I wouldn't defend any of these propositions particularly strongly. The primary motivator is simply that free people have an important historical right to arm themselves. This is true both in the Lockeian sense and in a much more ancient sense. I place great value on this right.