Thanks again for the help - I have done some more work on this and found that the two problems are unrelated.
The error message I receive needed a line of code updated as described here:
http://forum.abantecart.com/index.php/topic,5664.msg24484.html#msg24484The fact that the payment extension struggles with guest checkout customer_id being 0 still needs to be resolved but is an issue for the extension provider. I managed to temporarily avoid the problem by simply not writing to the database in question if customer_id is 0 - that lets people check out just fine as the database is not crucial for payment.
Thanks again for the help!