Hello,
let me start with my brief introduction, i have been working on odoo since version 4.2, it's been several years now. i have worked on each and every release from version 4.2 to v11, and now slowly starting few projects on V12.
i have worked on a variety of projects involving customization implementation of different modules such as CRM, Sale, Purchase, Inventory, MRP, Project, HR etc.
Related to this project
i have had implemented similar projects, where everything runs on odoo except e-commerce is on magento, and then odoo was integrated to download orders/customer info from magento
also there are some flags to differentiate e-commerce orders and offline orders, and journal etc configuration can be done.
Regards