I am so sorry you have lost the little one. Sometimes all we can do is make sure the passing to Rainbow Bridges is in a warm and loving environment. Sometimes with all the knowledge in the world the little ones cannot be saved. I will email you Cats Protections advice as to hand rearing kittens although I expect you know it already.
As you may know a mum cat will often reject a kitten if she knows something is wrong with it, so you may have been told the truth. As we in rescue are told so many lies I fully understand why doubts creep in.
I would also say that if you take on any more apart from the members who you have been advised to contact for advice Ela (and I am sure Leslie) has a good idea too as for 18 yrs there are not many times when she or one of her fosterers are not handrearing kittens. In fact we have one who came in a couple of days ago nearly dead but now it is coming on in leaps and bounds. Of course we know that this can change at any time. The only problem was the mum had no milk, the owner had been taken to prison and someone was going in twice a day to feed mum but did not know she had no milk, 3 kittens had already died.