I think there's too little data to make a meaningful statistical analysis. We've only had 55 changes in difficulty, and the first dozen or so were pegged at the minimum. There are a lot of people joining up, mining for a week or so, and discovering it takes a lot longer to generate bitcoins than they thought, and dropping out. Network processing power is quite volatile right now, and therefore difficulty will be volatile. We're looking at a minor decrease after a 37% increase on the next difficulty change.
I expect things will level off at some point, but I don't know when that point will be.
I'm not arguing any of your other points, I'm just saying it may be premature to try to correlate difficulty with price. But on the other hand, my investment history will show that I'm not a good predictor of the market (basically, if you do the opposite of what I do, you'd probably do very well :-D