[pauldodman]

It sounds like your problem - the 500 error - comes from the new payment method.


Suggest doing it one step at a time.
Put everything back - and just get the payment method working - so it completely generates the order.


Check the error log files - see if it is throwing a php error - but if it's a 500 server error you might not get anything.


How did you create the new quote payment method?
I would just take a default, offline one that you are not using, such as Phone Ordering and rename it.