I kind of came to the same conclusion, thinking that the oxide layer could only get so thick before it cuts off the oxygen from the metal pool underneath. So to get rid of some of the unwanted harder "stuff" in my lead mix, it looks like I'm going to try skimming and skimming and then skim it some more.
I've downloaded fryxell and applegates book from this link and am skimming some of the "more interesting to me" places
http://www.lasc.us/Fryxell_Book_textonly2.pdf
Seems to focus more on casting for the purpose of loading into a cartridge case but still has some interesting information on lead alloy properties. Looks like once you alloy it, it can be hard to undo...
Am starting to wonder about a borax based flux? From what I've read it looks like those tend to take everything out of the mix by creating an insoluble molten borate glass on the surface of the metal pool. Reckon that's more trouble than its worth or the path to softer lead?