> I’ve had multiple family members die with Alzheimer’s; can I sign up when young and lucid to die early if I get diagnosed?

In the Netherlands (and I guess other countries), you have to arrange this very early (if you know your family has Alzheimers, like mine). Once you have symptoms or even are diagnosed, you cannot decide anymore. And you'll end up in a home, knowing nothing and no-one and often drugged up, until you die of natural causes (although the last part you don't notice anyway; your family does).