Yes, you are correct that "hold fast against" in this context means to resist or withstand something. In the sentence you provided, "To use allegory to hold fast against the crushing blows of reality", the phrase "hold fast against" means to use allegory as a means of resisting or enduring the harshness of reality. Allegory is being used as a tool to help one cope with difficult circumstances. Here's another example sentence to help illustrate the meaning: The building held fast against the powerful winds of the hurricane. (meaning: the building resisted the strong winds and remained standing) In general, "hold fast against" is used to describe a strong resistance against something.

The others are correct. I don't want to dissuade you from using this language, but it is quite literary. It is not something a person would often say in conversation. However, most native speakers would readily understand the meaning.

"Fast" means "firmly fixed or secure". That meaning goes back thousands of years. It is still used in German ("fest") which is the predecessor of Old English. The modern meaning of "rapid" or "quick" has only been in the English language for about 600 years. To use it in a sentence, just write a sentence containing "firmly fixed" and then replace that with "fast".

Yes, it basically means resist. Maybe there’s a sense that you are doing what you think is right and not changing because others have changed.