OK- first, I must confess that I haven't read the work, but the piece you have written reads well. I'd change this sentence a bit:
Although, according to classicism standards (maybe just delete 'standards' or change it to the standards of classicism?), the writer should use lofty, sophisticated diction, Synge used the language of simple, vulgar (common\ordinary?)people selectively.
I'll come back later tonight when I have some more time and comment further.
