"Why does it matter what Russell said and did prior to then?"
It matters because he changed the date to a new one. The flawed vessel that postulated the first wrong date was the same flawed vessel that postulated the second wrong date. As a bad interpreter of prophetic literature, Russell had form. If he got his first date wrong, there's nothing to say that the second date wasn't wrong. He demonstrated that he wasn't a source of any special insight into the prophetic literature by championing a false first date.
James 3: "11 A spring does not cause the fresh* water and the bitter water to bubble out of the same opening, does it? 12 My brothers, a fig tree cannot produce olives, or a grapevine figs, can it? Neither can salt water produce fresh water." (NWT)
By getting the first date wrong, Russell proved himself to be a spring of flawed prophetic interpretation. His second date, coming from the same flawed source, is no more reliable.