I can find your first requirement is to have a solution that run on single admin panel, if I am not wrong. Second, you are expecting it will be open source. Now, I just wanted to ask you, Are these requirements for ecommerce websites or for normal websites? Search for Ydeveloper on Google with Multi Store eCommerce. We are one of the top multi store ecommerce solution provider. Our software is capable of running multiple ecommerce stores on single dashboard. This dashboard manages all the sections, inventory, shopping cart, orders, payments, checkout and shipping etc.
There are two resources I shared with you to learn more about Multistore Ecommerce, hope you would like to visit,
Solution - www.ydeveloper.com/multiple-store-management-from-single-admin-panel.html
Blog - multistore-ecommerce.blogspot.com