1.) Well, that’s the million dollar question isn’t it 
? No one can really say with certainty why it happens since there’s few ways to prove anything (I would like to point out that your original theory could even be right), so it’s really speculation either way. What makes the most sense to me having experiened FA many, many times is that it’s the expectation of waking up. Even if you’re unaware of it, the expectation can still be there. However, the body/SC are still in sleep mode, so transitioning to an FA is the best way to fulfill this expectation.
2.) Yes, that’s what I think on the subconscious. However, like I said before intentions and expectations are basically commands that can be given to the subconscious. If you haven’t already, I really recommend this article by BenDrummin and it’s follow up article.
Oh, and I thought you might like to see the epiphany I had on the subconscious. I actually thought basically the same things about it using DC’s or even FA to stop you from becoming lucid. Here’s what changed my mind.