Afaik Steam only takes the 30% if you actually buy it through the Steam store. If you buy directly on their homepage all the money goes to the devs. The provided Steam key will not cause them any additional costs.

What is the purpose of the priorities here? How would this differ in terms of throughput if the priorities were removed? They incourage the items to flow through the "bypass". Its necessary when the input is not full, but more than 1 belt. As an exmaple; If the inputs were on belt 1 and 2...