A frustrating experience. Try a different GM dealer.
From reading the posts, sounds like a fix may be needed. Wish I was closer and could help. Terminology hampers my perceiving exactly what you are experiencing, but it sounds like the top 'buttress' (if that is the correct term) on the left side is not releasing. That makes me think a release might only allow a temporary fix .... with a permanent fix still needed. The response from your dealer service person is unacceptable. That would put me in the dealers office.