Yes, you can use the US2 ducer as long as the graph is NOT an DI or SI unit. The US2 ducer is 2D only.
The reason it may not have worked with the 358 is that it is a DI unit, I am sure. It should work fine with a non DI 859 unit.
I don't understand what you are referring to with the i-pilot/humminbird GPS. There is no communication between the i-pilot and the Humminbird unit. To have a Terrova communicate with a Humminbird you need the "Link" feature.
I have a 958 that runs fine on my US2 ducer.