Personally, I believe the seating style in 0578 is the original style. Remember, When these hacks were originally built, they had 3 windows on each side. If the cupola seats had footwells, they would have interfeered with the middle window on each side.
When looking at the two main cupola carlines (the ones that are cut out for the aisle) it appears to be somewhat of a false cut out. What I mean is that one side seems to drop down for the seat, the other side seems to follow the roof curve, but about two inches down. It also looks like a piece was later added to attach the seat to. That is exactly the way that 0579's interior is. All this suggests to me that in the beginning these cabooses had a seat in only one side of the cupola.
Does anybody have any photos of the interior of caboose 49 at CRRM? That may help with the discussion.