I could be wrong, so take what I'm saying with a pinch of salt:
I think International Arrivals 1 is the old international arrivals hall, but they tend to use the new International Arrivals 2 unless the airport is really busy or if there's a long haul flight arriving.
From what I remember, the problem at Edinburgh was the lack of space, so when they built the extension a few years ago, it included new international arrivals and domestic arrival halls, and then International Arrivals 1 (the old international arrivals) took over the old domestic arrivals area including baggage reclaim as part of a new expanded arrivals hall. I also have the feeling that Edinburgh was hoping to use International Arrivals 1 as a place for USA pre-clearance, so there was no benefit in removing it, especially as Edinburgh doesn't really need the capacity at that side of the airport.
But generally, there's a mess at Edinburgh with the amount of buses used. They really need to just build a proper underground arrivals corridor for international flights and allow domestic passengers to simply exit through the departures lounge, rather than the current situation where flights can arrive at certain gates and then passengers (even domestic) need to be bussed halfway across the airport.