Reading about Murphy gave me chills, my Bob is nearly 10 years old and has always been 'his things' (including crate) aggressive, and can get nasty about holding on to his collar too. Has he ever bitten me or anyone in that regard? No. Because I know where the danger lies and so avoid it.

For example if I really need him to give up an item that may be dangerous to him (or valuable to me) I use some sort of distraction, perhaps hiding something he really wants behind a closed door. At this he usually drops the thing he's been holding on to and by the time he's found what's hidden I've retrieved the item, which he has already forgotten about anyway. And when playing with something, a ball maybe, and he drops it for me I always send him back a ways before picking it up.

But of course none of that is of any use to your sil. Knowing and living with similar dog I totally agree that Murphy would be a danger to a child. But I disagree that his aggression is unpredictable and he should be put to sleep. I am totally surprised at the rescues contacted in giving up on him. In my opinion, with training, in a new home with no children and a knowledgeable owner Murphy could be fine.
I only wish there was a rescue organisation equivalent to our Dogs Trust in your country, they evaluate and if a dog like Murphy cannot be re-homed they will spend the rest of their lives happily in their care. (I sponsor one such dog)