This argument does not hold regarding the rate against the dollar. The rate of supply of dollars is massively outstripping the real value of the debt they represent.

Lol. Citation needed. Maybe a graph of inflation or of the dollar devaluing massively against other currencies.

Except that this generation rate halving in price is a predictable, known event, and therefore already factored into the price of BTC.

I'd qualify as =might= be factored into the price of BTC. Whether it is or not depends in (large?) part on the financial acumen of people trading BTC and if they're bothering to model what the effects will be.

If you assume efficient markets, yes. Nothing I've seen about the bitcoin market so far would suggest that it's efficient. Low volume, ideological motivations, unstable technologies, few really safe places to store value, etc.

Oh, safely storing value isn't a problem: bitaddress.org.

Safely storing value while keeping it liquid, there's a problem.

How do bitcoins represent debt?

All money represents debt that your future payee "owes" you. It's basically a promise of future payment by "society" to you. There was a great Planet Money podcast about the origins of money, which was essentially promissory notes of farmers to provide X units of food in the future, but I'm having a hell of a time finding it now.