Piercing. Yes they can.
Yes. I have seen oral piercings completely covered in plaque. That means they are also a home for the bacteria that cause gum disease. They should be cleaned daily. Oral piercings are also likely to break teeth.
Yes. Plaque can build up on an oral piercing.
Infection haven. Any disruption of the skin layer is an invitation for bacterial infection. Plaque build up around a piercing is disease setting on the doorstep just waiting for the piercing to move disrupt the epithelial layer and invite all that --------into your body. Todays gratification is tomorrows mutilation.