The Serbs originally accepted the ultimatum.
Britain wanted to mediate. Grey saw himself as the arbitrator of a grand peace conference.
Source pleade?
It’s my interpretation of events as I read them either in John Keegan’s First World War, or The Guns of August. What’s to dispute? You even admit in your post that Serbia was agreeable to the Ultimatum, until Britain intervened.
Franz Josef was living large, yachting while the Ultimatum was delivered. The apology was going to be concluded all rather quickly until the bad news. With all the treaties involved, once the Ultimatum was declined, at the demand of the British, all the treaties created the domino effect leading to mobilization. At that time in history, armies mobilized and they went to war, there was no way to stop it.