Custom Order Processing Flow

[safius]

I am developing a store that needs a different order processing flow than the PrestaShop default. Specifically, it is supposed to allow users to put in their order regardless of whether they have yet logged in or not. The resulting page for the order submission assumes that all fields required for both the user account and the order submission are always present. If the user is logged in and they have an address set up, the fields are populated from the server data. If the user is not logged in, the fields are blank, so the user proceeds to fill them in, then they are processed accordingly by the server before the order goes through the system. Same thing happens with the address, if the user has not yet provided an address.

I am using the one-step process checkout option. On submission, all (10 or so fields in total) are passed as arguments to the server. Then I want to follow these steps in order:

if user is not logged in {
    If user account params belong to an existing user {
        log in
    } else {
        create user
    }
}

if address does not exist {
    register address
}

submit order

Validation is assumed but left out for simplicity.

1) What is the most frictionless approach to implement this?
2) How do I "submit an order"? Which controller does the functionality belong to?

Edited February 17, 2016 by Alex Giannakakos (see edit history)