What do you do when the patient has a gingival enlargement and you need to do a class 5 restoration? The gingiva would be covering the cervical third where the restoration is needed, so how is the tooth accessed?

This is not usually a problem. What I often do in a situation like you describe is a procedure called a "mini-flap". Basically I make two incisions apically from the mesial and distal line angles, reflect a small flap of tissue, and place a 212 rubber dam clamp to retract the tissue. The class 5 restoration can be accomplished in a normal fashion and when complete the tissue can be reapproximated and held with pressure for 5 minutes. Sutures are very rarely needed, the tissue will look normal within 2 weeks...