Presuming that you have an EC 261/2004 issue, just deal with IB through the standard claims channel. Who gets the matter within IB is an internal IB matter and your claim will not be resolved faster or "better" by varying the process.

As to other matters above, I strongly suggest that when a delay claim arises from an onwards connection, that you supply the specifics of that onward connection and what happened. When IB pulls up your information and sees that its flight either did not arrive sufficiently late so as to warrant compensation or that it was a Type 1 flight and you are seeking Type 3 compensation, you may avoid months of back-and-forth, by simply explaining your final ticketed destination, the type of flight, and thus how you arrive at the amount sought.