There's a large gap between removing all ambiguity and adding an explicit grouping syntax. Nevermind computers, plenty of English is far more difficult than it should be for humans to parse.

It would take much more than parentheses to render English "devoid of all ambiguity and shade". Even then, any such stylistic changes couldn't be mandatory (that's not how languages work). You already have the choice of fully explicating a thought using multiple simple, unambiguous clauses which are clearly separated. Authors may neglect this liberty for the sake of art; just as many would like to have that unambiguity, but neglect it because it is meticulous. And now you are going to argue against making parentheses more generally acceptable-- which would in most cases be little more than a stylistic change, favoring a grammatical construct which already exists in the language.

Furthermore, look to the Lojban community for examples of poetry and literature in a truly unambiguous language (a far cry from the English suggested by saulrh).