I take your point about how the appearance of darkness maybe due to a fine grain size in the nickel powder. However, the fine grain size means more surface area and more oxygen bound in the surface monolayer (or thin layer at least). The point is that any of the other primary reactants in the sealed tube: Li, Al, H are willing and able to replace the surface Ni and pull the oxygen off. Seems clear to me that the 10% LiAlH4 (whether weight or volume %) will react with all of the oxygen in the tube and do it exothermically. LiAlH4 is pyrophoric. In the context of heat generation this is more chemical heat to be subtracted when looking for excess/anomalous heat.
I also take the point that this is a critique of my explanation. Appearances can be deceiving in many cases: oxide layers on metals or water in clouds (liquid drops or snow?).
I return to emphasize my main point: Even if the vacuum furnace works by resistive heating, if the resistor is wrapped as a coil, it wil create inductive heating effects in the nickel. Took me a couple days before - smh! - inductive heating may well be the constant long term excess heat vs. the empty control tube. I like the elegance of the simultaneous control run. The experiment just needs an even more similar control.