Is the husband required to pay the full dowry to his Kitābī wife upon divorce, or is he to deduct from it what he paid for her insurance over five years?

What we understand from the question is that the insurance amount is now being deducted from the account. If this is in the form of a donation, it is not permissible to count it as part of the dowry (sadaq) or to reclaim it. If it is a loan, you have the right to demand it back. If it was intended to be deducted from the dowry, then you have that right. In case of dispute, recourse should be made to the Sharia (definition) court. We also point out that marriage to a woman of the Book, though permissible, should not be done by a Muslim due to the potential harms that may result from it.