We are in the process of creating a Salesforce Community, one of the function is to allow users to submit cases (enhancements, questions, etc...) to Salesforce Administrators.  We don't want to expose the Create Case permission to all the users and so we just to accomplish that with Visualforce page and custom controller.  There are two challenges we are facing.  If we use InputField element, the box is not available to users for enter input but the user is simply able to click Save to create a blank case.  If we use the InputText element, the box is available but clicking Save does not create a case and the debug log shows the following error message.  The error goes away if we enable Create case permission for the user.  Is there a workaround on this issue?  The user can create case as I mentioned so it's not a clean permission issue.  Can VF page be configured to ignore object and field permission just as in Apex?  Thanks in advance for any feedbacks!

|EXCEPTION_THROWN|[EXTERNAL]|System.VisualforceException: Create access denied for Case, controller action methods may not execute.

I am new to Visualforce and I am doing the Trailhead: Visualforce Basics Module - Use Standard Controllers.  The instructions state:
To preview your page in the context of Lightning Experience, open your browser’s developer console and enter:
$A.get("e.force:navigateToURL").setParams(
    {"url": "/apex/pageName"}).fire();
I am not sure where to enter this.  When I enter it in the Visualforce page the text just show up on the Visualforce page preview.  I must be missing something.  Any help will be greatly appreciated.