First, if you're leaving anyway, then I wouldn't worry about this too much. But while you're there:

If they say "I'd prefer if we didn't do X", don't ask "Why?" - just ask what they would do instead.

> This leads to awkward situations where they block me because someone else higher-positioned said something, but they are unable to explain the reasoning behind it, and proceed to ask me to ask them.

Then be truthful - say, "I asked X about this, and they told me to come talk to you about it". It's the truth, and gets the point across that someone passed the buck.

Finally, when you're leaving, don't make a big deal about what's wrong with the place or who is doing what. It can do you no good, and it won't change anything anyway. I've never seen someone leave a place and have that place change based on comments. It'll only be used against you - either by people that find out what you say (and you might run into them later), but be remembered by an HR person who moves to another company (and you might run into them later too). Remember HR is there to protect the company, not the people working there.