I believe there are two different concepts mixed in your response, and I suspect this is due to my question not being entirely clear (the Spanish–English translation probably didn’t help).
On one hand, there is the vendor registration process (store owners within the marketplace), who register through the form:
index.php?option=com_hikamarket&view=vendormarket&layout=form
According to my configuration, when they register, they are automatically assigned to the “vendor” user group and can see a menu item to access their vendor control panel.
On the other hand, there is the functionality that allows a store owner (a user with the “vendor” role) to add additional users to their store from their configuration page:
/index.php?option=com_hikamarket&ctrl=vendor&task=form&Itemid=664
(this is the screen I shared in my original screenshot).
Based on this, what I don’t understand is:
why is a store owner able to add new users to their store if the “special cases” you mentioned are not handled?
What is the current purpose or intended use of allowing a vendor to add users to their store under this limitation? Currently, this feature doesn't seem to be useful in practice.
Thanks