1) Is Bitcoin-Denominated Debt Enforceable?
In other words, say I lend someone X BTC (unsecured), and get them to sign a promissory note for the X BTC loan, etc... etc... Now, assume they didn't pay me back. Could I theoretically pursue them for this debt, and potentially take them to court? If so, would I receive only the USD value of BTC lent, or is there a way of writing a contract such that they would be obligated to provide me with the actual X BTC, no matter how much it is worth in USD terms, now?
Yes, there is. I've dealt with these agreements before. It's not as simple as just substituting BTC for USD in the text of the agreement, but it isn't impossible either.
Self explanatory.
It would depend heavily upon the state. Usury is a question of state law in the US.