Well, I got an eminent British war historian, author of many acclaimed, highly researched books, on my side. What have you got?
Keegan, in the quote I gave you, explains with precision how and to what extent, the Serbian stance evolved following the signals from the Russian Tsar.
You wonder why Serbia did not reject the ultimatum in toto. Obviously, it needed to appear reasonable enough to provide cover for the Russians' saber rattling.
Nonsense. You are engaging in historical revisionism.
There were only two options: War or Peace.
If Serbia wanted war, then it would have rejected Austria's ridiculuous 48 hour ultimatum (made after doing nothing for 2 full months).
If Serbia wanted peace, then it needed to capitulate to Austria's demands, which it did.
For its part, Austria didn't even negotiate. The Austrians had bet, badly, that Serbia would reject the ultimatum, and they had no Plan B if Serbia capitulated (which it did).
So Austria invaded anyway, even though Serbia's capitulation was so complete and well done that Germany's Kaiser William II said after reading Serbia's response that there was now "no excuse for war."
NO EXCUSE FOR WAR!
And that's from Austria's ally, not a neutral or opposed power.