If porcelain veneers are placed at or beyond the mucogingival junction, is this considered a biologic width violation?

Placemeny of veneers at the mucogingival junction is not a violation of biological width. Placement beyond the mucogingival margin could be a biological width violation, depending on amount of gingival attachment between the gingival margin of the veneer and the crystal bone level.  In your case, with what you have described, it sounds like a strong possibility that the biological width was violated.  I recommend that you have a consultation with a periodontist to get their opinion.  If indeed there has been violation, you would require periodontal surgery to re establish the biological width, and then replacement of the veneers.  

HI 
Unfortunately without an X-ray and photo it is difficult to give you accurate advice.  Please post photo and X-ray of the area.
Best,
Dr. Maddahi