This is how to set it up in konsole:
Also see the Kde Reference for Key Bindings in Konsole.
Settings -> Edit Current Profile -> Keyboard -> (select used keyboard layout) -> Edit -> Search/Filter for Return.
Edit the following entries (left column only):
Change Return-Shift-NewLine to Return-Shift-Ctrl-NewLine
Change Return-Shift+NewLine to Return-Shift-...
No need to install ibus, etc. All X11 apps have access to the exact keycodes and to their xkb / xim translations, and may ignore the latter.
The problem is in the terminal emulator, and with the fact that there's no standard way to represent key combos like Ctrl-Enter in the terminal. Also, each terminal emulator has (or hasn't) its own way of configuring ...
It looks like the Debian installer might use the well-known screen utility to implement multiple tabs on a serial console, so use its key combinations to switch tabs:
Ctrl+A, then 1-4
An alternative tool for the same purpose is tmux, which might be newer than screen... but apparently screen is a GNU project and tmux might be BSD. I would guess that screen ...