Unless I’m completely misunderstanding you, showing Thumbnails has an obvious button on the toolbar. Go to parent is harder, but I think for the sequence you describe there’s a back button on the toolbar that will do what you want. Otherwise, you can use the breadcrumb bar at the bottom (can’t remember if this is a default setting or not) or open the Go menu and learn that Command-Up is the shortcut for “parent directory”. (A little later you discover this is symmetrical: Command-Up goes “up” out of the folder you’re looking at, Command-Down goes “down” into the selected item.)

>Open a folder with images. How on earth do you switch to visible thumbnails?

It's really easy, or am I misunderstanding you? View -> as Icons. It even has a keyboard shortcut. Or press space bar while with the file selected.

To go to the parent, command click on the title in the title bar of the window. I don’t know how you’d ever guess that, but I remember it back from the Mac OS 9 days.