I would approach such a policy with care, you could be wasting your money. There are an awful lot of "no" in the proposal. It is called 'Flying Instructor Insurance' not 'Public Liability; Insurance. The ATO/DTO cannot contract out the liability that it has to it's customers and the world at large.. Nor can you insure the same thing twice. I cannot understand should a public liability claim arise, how the instructors insurance can apply. Not to have any such insurance can only be the ATO/DTO risk not the instructors. I can foresee though a very messy situation arising should a claim ever be made. Public Liability Insurance is not expensive for the ATO/DTO so smell a rat.
My advice is that you should join AOPA as an Instructor and obtain good quality professional advice from them. Insurance is never a straightforward matter and nor will be your legal liabilities.