The guarantor's signature is an internal regulation of insurance companies, and so is the back office signature. It is for some internal management of the company to prove who is responsible for this piece, and this person will be responsible if something goes wrong. The law first faces the responsibility of the insurance company, and then the insurance company will find the responsibility of the corresponding person. Therefore, their signatures also have legal effect.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.