Has anyone actually made a legitimate alphabet key thingy?

[nablator]

For (a) there is the "ch" ambiguity.

Yes, /ç/ and /z/ : You are not allowed to view links. Register or Login to view.

For other special combination of letters like "ei", "sch", "tz", "äu", I don't know if there is any reading ambiguity.

/ç/ can be written ch as in "mich" or g as in "schmutzig" so it has also a writing ambiguity problem (b). Then there is "v" and "f" also (b)...