Pessimistic assumption: Voldemort has made advance preparations which will thwart every potential plan of Harry's based on favorable tactical features or potential features of the situation which might reasonably be obvious to him. These include Harry's access to his wand, the Death Eaters' lack of armor enchantments or prepared shields, the destructive magic resonance, the Time-Turner, Harry's other possessions, Harry's glasses, the London portkey, a concealed Patronus from Hermione's revival, or Hermione's potential purposeful assistance. Any attempt to use these things will fail at least once and and will, absent an appropriate counter-strategy, immediately trigger lethal force against Harry.

Pessimistic assumption: Voldemort wants Harry to defeat him on this occasion. To get the best ending, Harry must defeat Voldemort, and then, before leaving the graveyard, identify a benefit that Voldemort gains by losing and deny him that benefit.

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".)

Pessimistic assumption LV has been planning exactly this conversation for months and has thought of every possible plan of action that he could do. He has Harry level intelligence. All viable solutions must therefore use information LV does not have access to, which does not include the fact that Harry is Tom Riddle. Asking for power he knows not is trying to patch this minor hole.

Pessimistic assumption: Voldemort has reasonable cause to be confident that his Horcrux network will not be affected by Harry's death.

Pessimistic assumption: Any plan which causes the occurrence of the vignette from Ch. 1 does not lead to the best ending. (For example, one reading of phenomena in Ch. 89 is that that Harry is in a time loop, and the vignette may be associated with the path that leads to a reset of the loop.)

Pessimistic assumption: The effect of the Unbreakable Vow depends crucially on the order in which Harry lets himself become aware of arguments about its logical consequences.

And not only Harry must not interrupt the game, he must prevent everyone else who do not know he's Time-Turned from doing it.

Pessimistic assumption LV knows that Harry can do partial transfiguration. LV has put up anti- apparition, anti- time turning and anti-transfiguration wards.

Less probable Pessimistic assumption these wards do not count as LV's magic once laid and will not resonate with Harry, meaning they will stay active. Alternatively, a death eater has laid them on previously understood instructions.

Pessimistic assumption: Voldemort can reliably give orders to Death Eaters within line-of-sight, and Death Eaters can cast several important spells, without any visible sign or sound.

Pessimistic assumption: Voldemort wants Harry to reveal important information as a side effect of using his wand. To get the best ending, Harry must identify what information this would be, and prevent Voldemort from acquiring this information.

Pessimistic assumption: Hermione, once wakened, despite acting normal, will be under Voldemort's control.

(somewhat shaky) Pessimistic Assumption

Voldemort can use a Time-Turner, too, and he will send himself a message from the future to win.

Concerning Transfiguration:

Pessimistic assumption: There are more than two endings. A solution meeting the stated criteria is a necessary but not sufficient condition for the least sad ending.

If a viable solution is posted [...] the story will continue to Ch. 121.

Otherwise you will get a shorter and sadder ending.

Note that the referent of "Ch. 121" is not necessarily fixed in advance.

Counterargument: "I expect that the collective effect of 'everyone with more urgent life issues stays out of the effort' shifts the probabilities very little" suggests that reasonable prior odds of getting each ending are all close to 0 or 1, so any possible hidden difficulty thresholds are either very high or very low.

Counterargument: The challenge in Three Worlds Collide only had two endings.

Counterargument: A third ending would have taken additional writing effort, to no immediately obvious didactic purpose.

Semi-pessimistic assumption: Harry is in the Mirror, which has staged this conflict (perhaps on favorable terms) because it's stuck on the problem of figuring out what Tom Riddle's ideal world is.

Pessimistic assumption Voldemort should not be killed, since without him it will never be known if the Prophecy came true.