I am so very sorry this has happened to you.
Be aware that, had she gone to just anyone, and not to a caring, concerned owner like you, the foal would have died in agony, so you have yourself to thank for living up to your responsibilities and doing the right thing.
Pity the worm that bred the mare to the stallion could not have done the same.....
I should just add here that, although in this case you were quite obviously right as there were clear indicators going on, it is NOT possible to diagnose a LWO foal by sight, even when tow LWO+ animals have been bred together...the result can still be a white foal and be normal and perfectly healthy..I only mention this because I would hate for someone to make such an assumption and euthanise a potentially viable foal before any other symptoms arose, thinking they were doing the right thing.
Even a Vet cannot look at a white foal and say it is LWO.
Only a DNA test, and/or waiting as you did to watch for other symptoms, can tell for sure.
Splash White and Dominant White both produce healthy foals that are white at birth, even Sabino, though rarely on it's own, can also do this.