Let's start with an understanding of the premise that anyone can be sued for anything at anytime. The only real protection that you have is the lack of a pot of gold at the end of the rainbow, e.g., no deep pocket or a case what has a very unlikely payout.
If someone is hurt or injured during training, you're in pretty deep kim che, unless you can show conclusively that the student willfully disobeyed instructions from you that they were perfectly capable of following (but he did if fine yesterday, or even 10 minutes ago doesn't cut it).
Once a student is "certified" you may still have to prove that the student was taught a given skill or knowledge point. More so than that, you maybe expected to prove that that student "mastered" it, and such "mastery" may not, in the jury's mind, follow the PADI definition, it may follow a dictionary definition, which is quite different.
The further in the past the course, the more con-ed the diver has had, the more dives the diver has made, all these will serve to lessen the likelihood of a big payout and thus will make taking the case less and less attractive.
Frankly, I think the only real solution to this issue is to get rid of liability insurance as we know it (not going to happen the agencies make way too much money on it in the form of "finders fees"), make boilerplate creation and servicing of LLC's for instructors along with "defense only insurance" available (Officers' and Directors' Policies). But don't hold your breath for such a reasonable product, the agencies make too much money off insurance, and get too much control through insurance to ever let go, they are, after all, concerned with their bottom line, not yours.