Conversely, unlike stocks, you don't need a broker, so there's nobody who would take on that dealer role.
Nobody has setup a "buy bitcoin on margin" service yet, and the first person to do so will lose a fortune to nonpayment of margin calls.
There also are brokers providing indirect Bitcoin shorting with 1:10 leverage in the form of CFD's (contracts for difference). Of course they could opt to always or sometimes not actually trade the coins - to their clients it makes no difference, as no actual coins can be moved in/out of the accounts.
Some possible ways that it might be interpreted to be more flexible are scripting, n-of-m transactions, and so on.
Reversibility shows up when you do transactions in a bank or other third party that can reverse the transaction on its own accord. There's no theoretical reason why this can't happen with bitcoin instead - you give your BTC to a hypothetical, highly regulated bank or broker or whatever, and then the transaction is exactly as reversible as any electronic transaction using dollars. The confusion sets in when you compare Bitcoin transactions with electronic transactions using fiat currency, when they're closer in many ways to physical cash transactions in nature.
I very much agree with you. This is also the right way to think about BTC exchanges -- an unregulated website that you ship cash to.
There are some subtleties around the specific nonphysical transaction mechanism of BTC that differentiate it from a cash transaction, which are sort of difficult to quantify currently because the technical and legal aspects have not been fully explored... as a hard example, imagine a BTC wallet coupled with a memorizable private key (or an effective substitute). This is essentially a cash store that cannot be confiscated, and which can be communicated verbally, i.e. within a protected (attorney-client) setting. There are some interesting implications there.