I have almost the same problem, but with a workaround that does not need an external keyboard. My Librem 5 arrived 4 days ago. I believe it has the Evergreen hardware, although nothing indicates that in writing. It runs the 6.0.0.1-librem5 kernel.
In Files, a file can be selected with a light (usually) tap on the file name. The file does not open. Longer tap brings up the right-click menu, which disappears before any option can be activated. Photon’s two-hand technique does not work on my device. Nothing changes after switching the Preferences>General>Action-to-Open-Items.
As others have noted, the simplest way to open a file is to bring up the virtual keyboard and tap Return while the file in question is highlighted.
For the other right-click options, there is a menu button in the top-right corner of the Files page. Tapping the menu button while a file is highlighted, you find a number of icons that activate Cut, Copy, Paste, New Folder and one or two others that I have not been able to decipher.
There are no words indicating what the icons represent. However, if you have access to a computer with Nemo or PCManFM-Qt file manager (e.g. Linux Mint or Lubuntu), the right-click menu on one of those will display almost identical icons along with the corresponding words Cut, Copy etc.