I am a Christian and I believe a general Christianity Q&A site will be very challenging to moderate because of the question of authority. To put it simply, not all Christian traditions rely on the same set of authority for answering questions of faith.
To give you an idea of what a moderator would be facing, here are some accepted authorities in general order of how universal they are:
The ancient creeds—in particular the Nicaean and Apostles' creeds. One or both of these are accepted by all Christians. However, there are religious movements (not naming names) who call themselves Christian, but either don't accept the creeds or don't seem to take them seriously.
The books of the New Testament. While the canon has been set since about the 2nd Century, there are still people and movements who add or subtract from the standard list. I suspect that soon, if it hasn't already started, there will be Christians suggesting adding various Gnostic writings to the canon. Major theologians have at various times suggested removing books (most commonly James).
The Hebrew Bible. While there is minor controversy surrounding the canon (most Christians don't accept or rely the books of the Apocrypha, hence the name), there is considerable controversy about how Old Testament applies. Most Christians believe that Jesus established a covenant that supersedes the covenants God made with Abraham, Moses and the nation of Israel. For that reason, it's usually not strictly applied. But sometimes the kosher rules and so on, are held to apply by certain traditions.
The Holy Spirit. Theoretically, the Holy Spirit is an accepted authority by everyone who accepts the creeds and the New Testament. One the other hand, and this connects directly with your second question, some traditions hold that some manifestations of the Holy Spirit mentioned in the Bible no longer occur and others hold that additional manifestations are common. Finding common ground on this authority, sadly, has been nearly impossible.
Church tradition. I'm showing my bias by putting this item so far down the list. Some traditions have traditionally held that ancient church traditions can override even scripture in certain cases, if you get my meaning. Other traditions find little additional authority in church fathers because they were guided by some combination of the creeds, scripture and the Holy Spirit, which are all still available to us.
Church authority. When you boil it down to root causes, almost every church division results from disagreements over who has authority within the church hierarchy. At this level, nearly all appeals to authority will produce more heat than light. Basing a Q&A site on one particular of these authorities (such as Roman Catholicism) seems like it might work, however.
Personal conviction. In our post-modern world, personal conviction has become a source of authority, not just within Christianity, but in most "religious" topics. Personally, I'm convicted that most examples of appeals to personal conviction are attempts to avoid conviction that arises from one of the other sources of authority. (Yes, I see the irony here.)
If a Christianity site is established, I strongly suggest searching for an agreeable set of authority which is laid out clearly in the FAQ or such. The list above minus the last 2 or 3 items may be a good start. Another approach would be to adopt the position C. S. Lewis took in Mere Christianity, a recent statement of our common belief.
As a more direct answer to the type of questions proposed in this question:
I believe that either the site would need to establish a framework for answering questions or the questioner must provide a framework that they would agree to. For instance, the Greater Catechism asks:
Q: How know you [the Bible] to be the word of God?
A: By the testimony of God’s Spirit, working faith in my heart to close with that
heavenly majesty, and clear divine truth, that shineth in them.
For some, the authority that answer appeals to is valid and for others, it isn't. But within it's own framework, it's a valid answer.
Another framework might be to ask how the particular books of the Bible became be seen as divinely inspired works. That would put the question in the framework of history and church tradition which might provide more satisfactory answers.
The later question could be answered if the framework of the Bible or the authority of some particular church were chosen, but would be a poll if no more specific framework than personal experience existed.