Ran into a problem testing this against the authorize.net sandbox. In both SIM and DPM modes, according to the log, the transaction failed with an MD5 hash check. No hash was set in the module preferences, as there's no way to configure the hash on the authorize side for the sandbox, so it shouldn't have been checking this at all. Your previous version didn't exhibit this behavior, for what it's worth. Can you duplicate this?

Couldn't get far enough to check, but has the SIM mode reverted to the behavior where a customer can bail out on the relay response form without clicking to return to the site, and thus avoid the cart/checkout/complete processing? Avoiding that situation was point of enabling relay response for SIM - folks were successfully paying but their payments weren't recorded.