Your presumption is incorrect, Apple do not get a cut of Apple Pay transactions.
8 publicly visible posts • joined 14 Jun 2008
The Unfair Terms in Consumer Contracts Regulations prevents them from doing so. Orange have two choices - either they don't have the abillity to increase the pricing at all (during the term of a contract), or they do have the ability to increase prices, but they must offer the consumer the option of cancelling.
If the outgoing admin is slightly more imaginative in their backdoor-creation than "add a new Administrator account" then you are going to be hard-pushed to find it. So no, this couldn't necessarily have been avoided, and "locking the system down" is not possible in the way you suggest.