Btw if I use French input source while using English keyboard, do you know a way/shortcut to briefly show keyboard viewer in different language than the active input source so I can see quickly where is a certain character I might not be sure about?
Maybe this last idea: In the cases where you need only two different characters, eg a and à, you can use a key sequence. eg a typed twice = à.
Typed once (normal) = a.
If you need aa often and don't want it to become à automatically, use a sequence with aa+space. This should eliminate interference.