So it is only certain items that are failing currently? To get a better idea of why it is failing you should add the full request and response to your failure page so that you can see what the error is.
On your failure page select the part of the page you would like to see the info then go into code or split view and add in this code:
The full request was: <?php echo(AuthNet_Result("Full Request")) ?><br/><br/><br/>The full response was: <?php echo(AuthNet_Result("Full Response")) ?>
Please post back with this info minus any of the sensitive details like the cc info. This should let us know what the problem is.
As for the login you can add in whatever descriptive text you need to inform the users that they need to be registered and logged in to proceed with the checkout.