Hello here are the answers:
1. I see that your Component haven't a Front End Item submission. There is no frontend submission, you can give proper access to any user and name your backend like "backoffice" or "dashboard" where seller can login to add products.
2. You give to Vendors Back End Dashboard Permission? its not exactly backend, sellacious is entirely independent application so you do not give user /administrator access you can define permission in sellacious backoffice can rename "/sellacious" to anything like "dashboard" by simply renaming the folder and sellers will login there to add and edit products.
3.Dashboard haven't button or link go back to site Frontend. yes there is a button in the backend to go to frontend, every product in backend has button to see that product in frontend and on frontend has the button to see the product in the frontend
4.Categories of Products are together with Categories of Vendors. Category ars a horizontal module, any profile using categories is managed from the same module with different options based on category type.
5: if your Component has integration with Joomla ACL? Yes, we are integrated with Joomla ACL, but to make it easier to work, permissions can be managed from sellacious back office under the settings menu, we recommend you to use sellacious permission page because slowly and gradually we are getting independent from Joomla administrator UI.
Let us know if we can help you any further.
-Abhishek