Last time I made a batch I boiled the resulting liquid down to a dark almost black syrup then added Isopropyl alcohol to it for bottling. It does still make a lighter shade of walnut compared to the Fiebing's Walnut Dye.
If you apply liberal amounts of paint to the lettering depressions then wipe the excess away with a square sponge or towel on a block, it leaves paint in the depression. Similar to applying antiquing paste.
Will Ghormley uses a water based rust tank to make grey to black leather: http://www.willghormley-maker.com/MakingHOGRig.html (scroll down)
Probably smells much less...
Those period buckle clips are very hard to find indeed!
Do you know someone who can fabricate it from a small piece of brass sheet such as a silversmith or jeweler? Anyone who works metal should be able to do this for you.