What is the command line shortcut to comment out the current line

Documented in the bash manual here:

M-# or AltShift3

Home # Enter works obviously, and is quite short. An alternative to Home is Ctrl+A.

Another possibility that might be closer to what you want in practice is to clear the line with Ctrl+U, which stores it in a buffer which you can recall with Ctrl+Y within the same terminal session. This won't save it to history, but from your use case it sounds like you want it within the same terminal session anyway.