I am considering a long-term switch from the Cinnamon desktop to KDE Plasma. I am now only an occasional KDE users via virtual machines installed in VirtualBox.

To follow through on the switch, I'll need:

[*] An easy way to switch between the U.S. English keyboard and the English-with-international-dead-keys keyboard. in the international keyboard in Cinnamon, I can enter characters like é, á, í and ñ by holding down the right Alt key and pressing the appropriate letter. I've hunted online, but haven't found a way of doing this easily in KDE. I don't see the function in KCharSelect.

[*] A simple file launcher. With Cinnamon's Places Center, I can pin a file name -- invented example: /doc/work/meetings/notes_from_august.odt -- to the menu, and pull it up in the default LibreOffice program just by clicking on it.

KDE's Application Launcher looks sophisticated, but AFAIK lets me pin only applications, and not files. I see a Places Widget in the KDE store -- https://store.kde.org/p/1084935/ -- but wonder if I'm missing the main app used for this function.

Thank you for considering these questions from a KDE newbie!

For the keyboard layout switching, you should be able to configure that through System Settings -> Input Devices -> Keyboard -> Layouts.

For a menu which presents a selection of favorite Files, you could use a Folder View or Shelf widget pointed to a folder containing links to the various files.
One caveat with references to Places in KDE is those are generally tied to the Places panel in Dolphin and must be directories (though they can be local or remote, as made available through KIO), so you couldn't use that Places widget for documents as per your example. Though if you set up your own favorites folder and use a Folder View or Shelf widget you can put (links to) whatever you want in there.

Thank you, airdrik!