We're ignoring the situations when the host picks the winning door, because obviously you lost.
We're focusing only on the times the host "accidentally" picks the losing door.
It doesn't matter how "often" he picks the losing door. Because in the cases when he doesn't, we don't care about.
We're looking at the situation of Guy picks door A. Host opens door B. Door B is empty. Should Guy switch to door A or door C?
The host's intentions DO matter, if you look at the big picture and factor in amount of times that the host reveals the prize. But we're not doing that, because the original question stipulated that the host opens an empty door.
All of your factors are basically the 2/3 chance the dude has of winning, multiplied by the chance the host has of making him lose instantly by revealing the prize. That is all well and good, but in the scenario given (where the host DOES open an EMPTY door), it's entirely irrelevant because we are ignoring those situations.