Generally speaking, should off-site links be edited and replaced with ones on the Stack Overflow network?
For what purpose? In an ideal world, the content on the Stack Exchange site (be it a question or an answer) should be whole and complete. Links to external resources can be used as reference, but should not be used as content. If an SE post says something like, "click here to see an example of what I'm talking about" then it should be edited to include that example, not edited to link elsewhere.
- In the situation that a SE answer and a blog offers equally unbiased valid technical advice, which one should be linked to?
That's up to the person who posted the content, I suppose. You're certainly free to edit any questions/answers as you see fit. Perhaps adding the link to the SE site to provide further context for the question/answer would be a good idea. But mass-replacing people's links with other links is probably a bad idea. You run the risk of changing the meaning of the content for little purpose other than to try to internalize all knowledge to Stack Exchange.
- When should SE links be given preferential treatment?
I'm not sure what you mean by this. Preferential treatment for what? A link is a link is a link.
- Should older questions be edited to point to an SE answer if it offers more up-to-date valid information?
From what you're saying here, it sounds to me like one of these questions is a duplicate of another. If someone asks a question on a Stack Exchange site and the answer to that question consists of a link to another answer, the question itself should probably be closed as a duplicate.