quicktime list

How can I clear that list?

If El Capitan works the same as Sierra, then

1. Open the QT X Player app and use the File Menu "Open Recent > Clear Menu" option to clear the File Menu "Open Recent" list
2. Close the QT X Player app
3. Right-Click the QT X Player icon in the dock and select the "Options > Remove from Dock" context menu option to clear the Dock "Recents" list
4. Open the System Preferences... "General > Recent items" window and set your preference for the max number of items to be displayed in the Dock "Recents" list (I.e., None, 5, 10, 15, 20, 30, or 50)
5. Add the QT X Player icon to the Dock Bar to store your Dock "Recents" list display preference
6. You can now reset the System Preferences... "General > Recent items" window preference to control the max number of recent items to be displayed in the QT X File Menu "Open Recents" option (if different than the Dock max display default)

Basically, the current System Preferences... "General > Recent items" window setting controls the number of recent documents/files to be displayed in the QT X Player app's File Menu "Open Recent" list while the current System Preferences... "General > Recent items" window setting in effect when the Qt X Player app's icon is added to the Dock controls the max display limit for recent items displayed in the Dock context window.

For one thing, there are so many listed there that it no longer lists the most recently played videos, just those I watched months ago.

Sounds like your dock was initialized with a 20, 30, or 50 setting. I normally use a global "5" or "10" setting for all operations.