I'm not sure if it's an error with discuss, but I don't know what the why is in relation to because my comment you have responded to is not a negative.
If are you asking why not grouping - it's because I just couldn't recall ever using this. However, I have just realised that in Australia we do have a similar system that I just hadn't used it - it's called the ABA . So does this mean that a group corresponds to a file with a list of payments?
I understand that Tryton allows the creation of payments and does not link them to invoice ( in regards to the module account_payment and the payments model ) - to me this seems wrong and this is why I am not sure about using the account_payment module as a "base" for a payment gateway module.
Good point about the invoice generated only after a payment is fully accepted, but I don't have experience of this. The payment is usually accepted as successful, although usually it will go through a "clearing process".