Couldn't some of these problems be easily worked around if the popular exchanges offered an API that generated send-to addresses that would auto-sell at market? For low volume simple cases anyway.
eg. You put an image in your shopping cart that is generated from such an exchange API call. It says "Send 1.05BTC to 1ze656..etc" and that address is linked as "auto-sell" to your exchange account. And ideally the exchange sends a confirm email.
Now when a buyer sends that BTC there the merchant simply checks his exchange account that the USD were received, or believes the email confirm, and approves the sale.
Sure, it's somewhat manual but it's a simple feature that could help many low volume merchants accept BTC when they can't get full cart integration. And it improves liquidity for the exchange offering it as it will bring in trades at current market price.
eg.
is all the merchant needs to add much like a paypal button. The image gives amount in btc and address. The exchange tags that address as "auto-sell at market to this acctcode". It's a "magic address". Magic Computer Money should have magic addresses.