Answering the individual questions
Note that these "answers" are informal and reflect what I have seen occur in the past with these types of situations; they do not reflect what has always happened or will always happen, only what I would call the "typical" results:
Is there any kind of official guidelines that govern policies?
Aside from the following Q&A I am not aware of any rigid (or loose) guidelines regarding policies; only what generally happens to happen:
What determines if a policy is valid or not?
I'm not entirely sure what you mean when you use the word "valid"; just about anything can be a policy if it has (Meta) community consensus.
How do votes interact with policies?
Often, policies stem from questions about policies. That is, a question will ask how/whether something should be done and answers will give differing, well, answers to that question. The highest-scoring answer is likely to be considered the "policy" answer, as it is the one with the most (Meta) community consensus.
How are contentious policies handled?
Policies can be revisited after they are made (I at least believe that they should be as otherwise it would be tradition for tradition's sake and current policies wouldn't reflect the current state of the site). That said, how often policies can be revisited is not something I have seen answered anywhere.
Note that contending with a policy does not change the fact that it is the policy and that it must be followed.
Is a majority needed to implement a policy, is consensus needed, or something else?
The highest-scoring answer to a question about a policy is the one that is the most agreed upon by the (Meta) community (or at least something similar to being the most agreed upon answer). Thus this is the answer that is, for lack of a better word, "promoted" into being a policy.
Are moderators be allowed to overrule policies?
Well, I'm not a moderator, but moderators are site users just like everybody else and they (as far as I'm aware) cannot assume any sort of dictatorial or "because I said so" sort of stance. They must follow policies just like regular site members.
What happens if a user does not follow a policy?
Ideally, they would have the current policy explained to them and would learn from it moving forward by following the policy in the future. Whatever policy-defying action they took would almost certainly be undone but I certainly don't believe people should expect users (especially new users) to know the various policies of RPG.SE which are buried across numerous posts on an almost completely separate site.
Do policies get less valid with age?
I don't believe they get "less valid" but they certainly get less reliable in the sense that they reflect less the current state of the site. If a policy stood for 50 years one would certainly hope that the reasons the policy was ever implemented were still around and if they weren't still around, one would certainly hope that the policy be revisited as the times had changed.
How long do policies last for?
There is not set time that I am aware of. Policies are changed by being revisited, so without a revisit I suppose they would last indefinitely (unless of course the policy came with an expiration date, which I have never seen happen).
How are contradictory or conflicting policies handled?
These should not ever exist. If two policies contradict one another this is a fundamental problem that should be pointed out and addressed quickly enough that the contradiction has little to no effect on the sites.
Can a more specific policy overrule part of a less specific policy?
Without a specific example this is probably unanswerable. However, as far as I'm aware, policies can do whatever they want (while avoiding contradictions); after all, it's just an agreed upon set of rules.
When are policies revisited?
I don't know. There is probably an entire separate Meta discussion buried in here that I don't want to hash out.
Does revisiting a policy invalidate the original?
The answers and votes and comments and time that users spent on the original question are still noteworthy, valuable, and available; so it isn't invalidated in that sense. However, yes, the entire point of revisiting a policy is to either change or maintain the then-current policy (you could call this "invalidating" it).
How should users determine what a policy is?
Generally if a question is asking about how something should be done or whether something should be done; the highest-scoring answer is going to be the current policy. That doesn't mean every Meta Q&A is about policies (we probably have very very few actually policies at all), and it is only the case generally. Recently, the moderators have gone and added little sections of text to the end of some policy questions describing what the current policy actually is in case it would/could otherwise be unclear.
Do policies need to be followed all the time, or just some times?
The entire point of a policy is that it is always followed by everybody. However, context, context, context: It matters and shapes our ideas around things because we know a policy can't be perfect and account for every possible future event, I mean, we can't even accurately imagine every possible future event so we have no chance of accounting for them all anyway. We just do our best here and we know situations and events can occur where a "policy" gets... complicated.
I at least do not want a rigidly-defined process
To me, having a rigidly defined method of creating and establishing policies (such as through implementing a set time after which the highest-scoring answer is promoted into a policy) gives easy ways to track down errors, loopholes, and ways to abuse or cheat the system.
A quote that summaries much of how I feel about the "policies" here:
[...] I wish the word "policy" weren't getting slung around so much. Like it or not, we don't really have policies nearly as much as we have practices. There are very few things around here that anyone--editor, voter, moderator, employee--is going to say is so clear that "yes, this will be applied in all situations, no consideration given, no context needed."
We aren't a legal system, we aren't a rigidly defined system of checks and balances. We're people trying their best, and we know that a rule can always have an exception. You simply cannot see the future.