Pessimistic assumption: An intended solution involves, as a side-effect, Harry suffering a mortal affliction such as Transfiguration sickness or radiation poisoning, and is otherwise highly constrained. The proposed solution is close to this intended solution, and to match the other constraints, it must either include Harry suffering such an affliction with a plan to recover from it, or subject Harry to conditions where he would normally suffer such an affliction except that he has taken unusual measures to prevent it.

(This is one reading of the proviso, "evade immediate death".)