Question : Does the seller needs to create an account in MangoPay or is it only the Admin who needs to create one?
Answer : To use Magento2 Marketplace MangoPay Payment Gateway only admin needs to create an account in MangoPay. Admin can create an account in MangoPay from here. Seller just needs to provide the bank details at seller panel to transfer amount to his bank account.
Question : How the commission paid to the Admin? Is it automatically split?
Answer : Yes, the amount will be automatically split between the admin and the seller.
Question : Who pays the fees? The seller, the admin or both?
Answer : The admin pays the fees. You can check the MangoPay fee structure here.