However, I did discover that unchecking "Show Content Guide at startup" in Player Preferences resulted in the normal player controls to show up, so that fixes it. I don't know why the content guide will not display but at least I can work with it as it is now.
On a Mac the issues you described might be caused by a corrupted PLIST file. The normal "fix" is to close the QT app, delete the PLIST file, and reopen the app to force re-generation of a new PLIST file. Such problems can normally be tested beforehand by simply opening the app under a different user account since each user account has a different PLIST file associated with it. If QT works properly under one user account but not another, the issue is usually a corrupted PLIST file causing the problem whereas if all accounts have the same problem(s), then it is usually the App itself or a conflict with the open QT app that is causing problems.
So has Apple gone to user-based support only now? Even for paid software? Not being able to file a trouble report or bug report with Apple is, well, troubling.
No, but usually it is easier/faster to find assistance here in the forums. Use the Product Feedback form to file bug reports, provide official feedback, and/or request enhancements to a product. See the https://www.apple.com/feedback/ page to find a list of links to specific product feedback forms.