Although I don't know how (or whether) to fault the 'Who is clever...' form, it certainly doesn't sound right - except possibly in a context where 'I'll tell you...It's....' is implied.
Incidentally, re your last examples, some tricky collocations: 'He must be mad' (probability) but 'He should be shot' (hyperbole). Saying 'He must be shot' sounds as though you're issuing an order (compulsion, not probability!)
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