@guanophore, I will attempt to answer your questions about methodology in this post in one go. Apologies in advance for my shortcomings.
You’ve asked a series of excellent questions on sourcing and ancient historical methodology. In an attempt to keep this post to a reasonable size, I will have to summarize some concepts that ancient historians have been debating for over two centuries. As you can imagine, thousands of books have been written on the subject.
When trying to evaluate ancient sources we typically only have fragmented data on the source. Take, for example, the Gospel of Mark. There are some very critical things we don’t immediately know: the gospel is not signed (no author), there is no publication date given, since we don’t have the earliest version - we don’t know what language it was written in, it doesn’t say where it was written, etc. Scholars used a wide array of methods to try and come up with reasonable answers to these questions. That array of methods is sometimes called “higher criticism,” but more typically just “criticism.” Please note that the word “criticism” isn’t being used in a pejorative sense, it is more akin to “evaluation.”
The forms of criticism that exist under that umbrella are far too broad to explain in a forum post, but you can find some articles on higher criticism or Biblical criticism if you are really interested in the gritty details. In brief, taken together, these techniques allow historians to attempt answers to questions about the source material that otherwise, we would just be guessing at.
Take for example the question of the ending of the Gospel of Mark. The earliest manuscripts we have (from roughly the late 300s), end at the verse ending in “they were afraid.” They have no mention of resurrection appearances. Only later manuscripts contain resurrection appearances. Through a variety of methods - particularly by examining the precise language used - historians broadly agree that the new longer ending is not original. The debate is now focused on whether there was an earlier ending that has now been lost, or if Mark really intended the gospel to end without resurrection appearances.
