How to enforce users to input user account's fields after user registeration?
Try using Rules. You could add a new field into the users' accounts that is Completed Profile Fields and it could be a single boolean or possabily a multi-value field.

Then your rule could check:


If login, or page load -- has user completed profile fields? If yes, continue to any page -- else show a profile form page (not necessarily their account page).
I'm not sure it would be easy to do this globally, or test the profiles have been filled prior to going to a /node/add page for instance.
After filling in the profile values the Rule could update the user account field completed profile fields to True.


If you don't want users to see this completed profile fields field -- you can install and use Field Permissions.

EDIT

If you dont want the user to see this special completed profile fields but still want the rule to update the value you may need to set the uid of the user updating this field. I do this usually by creating a fake user for the system called like mysite-system-user and give him a special role called system role for instance.