"They can complain because they do not, by wearing such garb, intend to withdraw from society"

I've generally agreed with you here but I disagree on this point.  I think the niqab most certainly *does* signal a lack of willingness to engage with others, just as a punk wearing a t-shirt saying "F off!" does.  That person is sending me a message with their garb.
That said, nobody owes me engagement, and nobody is required to identify themselves in order to ride the bus.