sql server 2012 Express bulk-insert fialed due to message Operating system error code 5(Access is denied.)
Run SQL Server Configuration Manager and under SQL Server Services find your SQL Express instance and see what the Log On As is set to. Alternatively, you can right click the server name in SSMS, Facets, then scroll down to "Service Account" to see it.

If it's the default NT AUTHORITY\NETWORK SERVICE then grant access to a computer account called YOURDOMAIN\YOURCOMPUTERNAME$. You may also need to grant access to the person executing the command if they're logged on using AD.
Regarding your other user-level access issues, if a user is part of the db_owner role as shown in the screenshot they have the highest database-level access possible and no other database roles are needed. And if they're the database owner (separate to db_owner but pretty much the same privilege wise) then you can't grant additional database-level rights nor do you need to.