The broker is correct. You should get most of your documentation from the actual juice supplier. The broker is the one to be the middle man in your communication concerning this. However, the broker does have responsibility with FSVP and insurance as they noted.
The letter of guarantee and food safety documentation should come from the juice supplier, not the broker. The broker needs to verify the supplier has met the FSVP requirements. See the link below from FDA regarding FSVP requirements and how to meet the requirements. FYI, the broker is considered the importer, not you.
Our juice broker tells me he is only able to provide proof of insurance for his broker company and he can provide supplier documents from the juice manufacturers. The broker does not import any of the juices we purchase they are just the "middle man" between us (customer) and manufacturer. I'm thinking they should be able to provide some sort of letter of guarantee or something other than just a COI. Any thoughts?