I do have Ctrl under ä (which would be the semicolon key on US layout, I think). Interestingly, on Mac (with Karabiner) it caused regular mistypes when typing fast, even after a year. On my 12 year old Thinkpad (Linux with keyd) however, I’ve never found the overloading to be an issue.

I’ll probably give the layout in the article a shot. It sounds interesting.