It is an interesting point you make about about William being out with Alan that morning.
In Rifle and Spear I use a little artistic licence and write a short fictional piece about Alan shaking hands with William before he rides off into the mist with Chelmsford's column on that fateful morning and I've always assumed that William remained in the camp.
He was a civilian, not mounted infantry,so I don't think he would have gone out with a fighting force. Other aids of senior officers, such as Private John William's, Glyn's groom were left behind.
However as we know so little about him, he could have been a retired Hussar or trooper from the Derbyshire Yeomanry that Alan got to know during his stint with them. If he was he may have been more likely to have gone out with them to 'have a bash'.
Alan makes no mention of William being with him out at Mgogo and as soon as Alan returns all hell breaks lose and I think its highly likely he never saw him again.
As a civilian he would certainly not have been with Alan when he took Bradstreet's men out in his gallant action to shore up the British right flank nor when he valiantly tried to pull a scratch force together and get through to Lord Chelmsford.
I can imagine Alan, that morning,telling him to look after his tent and spare horses and that was the last he saw of him.
Although I can't prove it 100%, I would say theres more than a fair chance he was left behind in the camp.
Unless more information comes to light we will just have to speculate.