I believe you’ve confused mutli-boot (in the sense of multiple operating systems on internal disks) with portable USB installations. The latter is explicitly supported for general use by end-users, see:
This is likely unfortunately a limitation of LXQt 2.1. We’ve tried to create tools to fill in the missing gaps, and will continue to do so as needed.
This is documented at:
In short, boot into sysmaint mode and run sudo set-system-keymap --interactive, then follow the prompts. You will want to use something like us,de for your layout settings, and the option grp:alt_shift_toggle or similar to allow switching layouts by pressing a key combo (in this case, Alt+Shift).
Yes, that should work, in particular you will want to follow the directions listed under “UEFI firmware fails to pick up Kicksecure boot entry”.