It looks like EZConnect can't connect for whatever reason (3a works/3b doesn't). You can try adding "ezconnect" to NAMES.DIRECTORY_PATH in the sqlnet.ora file. Hopefully that will make 3b from @Albert Godfrind's answer work.
The LDAP connection may fail from QGIS because you have something like the Oracle Instant Client somewhere in the path before your C:\OraClient11g\ client. In that case, sqlplus and tnsping would get loaded from the C:\OraClient11g\ client, while QGIS would load the other client that is missing the sqlnet.ora file. You can set the the variables ORACLE_HOME and TNS_ADMIN to tell the Oracle client to load the admin directory that it specifies, rather than the default relative path. Try setting these variable to force Oracle to use the same admin dir that sqlplus is using. Start QGIS from the command line after setting:
SET ORACLE_HOME=C:\OraClient11g\product\11.2.0\client_1
SET TNS_ADMIN=C:\OraClient11g\product\11.2.0\client_1\NETWORK\ADMIN
NOTE: I'm not sure if TNS_ADMIN is going to be just for TNS Names looks up, or if the client will load the sqlnet.ora file and use LDAP. Technically, I think ORACLE_HOME will be enough, and you don't actually need the TNS_ADMIN variable or tns_names.ora file.
I got this to work with TNS_NAMES by setting the TNS_ADMIN variable, then starting QGIS 2.4.0. I created the connection by only setting the Database, Username and Password values in the "Create a New OGR Data connection" dialog.
The version of QGIS that I have does have the instant client included in the bin folder, so you wither need to set TNS_ADMIN to use tns name conenctions, or create the Network/admin/ folders in the QGIS bin folders and copy your tnsnames.ora and sqlnet.ora file into it.
It looks like if you set the Host values in the connection dialog, QGIS tries to use an ezconnect connection.
User Profile Management was added in QGIS v3.0.0. Some information may be found at the GitHub
User profile contains information about the user profile folders on the machine. In QGIS 3 all settings, plugins, etc were moved into a %APPDATA%/profiles folder for each platform. This allows for manage different user profiles per machine vs the single default one that was allowed in the
past. A user profile is all settings and anything that used to be found in .qgis3 in the users home folder.
The management tools can be accessed from the Settings menu under User Profiles.
Functionally, this implementation acts in much the same way as the process described in this question and the comments. Loading a new user profile from the menu will open a new instance of QGIS using its associated profile folder.
Merely copying any profile folder to a target installation environment will achieve the migration of settings, but this does not address a "centralized" deployment.
Best Answer
The settings on Windows are stored in the registry:
HKEY_CURRENT_USER\Software\QGIS\QGIS2\Qgis
As Jakob said, you could convert to using an ini file and start QGIS with the appropriate option to use it instead of registry settings.