Pessimistic assumption: Voldemort evaded the Mirror, and is watching every trick Harry’s coming up with to use against his reflection.
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 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 has reasonable cause to be confident that his Horcrux network will not be affected by Harry’s death.
A necessary condition for a third ending might involve a solution that purposefully violates the criteria in some respect.
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: 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: Free Transfiguration doesn’t work like a superpower from Worm: it does not grant sensory feedback about the object being Transfigured, even if it does interpret the caster’s idea of the target.
Pessimistic assumption: At least in the limit of unusually thin and long objects, Transfiguration time actually scales as the product of the shortest local dimension with the square of the longest local dimension of the target, rather than the volume. Harry has not detected this because he was always Transfiguring volumes or areas, and McGonagall was mistaken.
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: Hermione, once wakened, despite acting normal, will be under Voldemort’s control.
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: Voldemort, and some of the Death Eaters, have witnessed combat uses of the time-skewed Transfiguration featuring in Chapter 104. They will have appropriate reflexes to counter any attacks by partial Transfiguration which they could have countered if the attacks had been made using time-skewed Transfiguration.
Pessimistic assumption: It is not possible to Transfigure antimatter.
Pessimistic assumption: Neither partial Transfiguration nor extremely fast Transfiguration (using extremely small volumes) circumvent the limits on Transfiguring air.
Pessimistic assumption: Plans which depend on the use of partial Transfiguration, or Transfiguration of volumes small enough to complete at timescales smaller than that of mean free paths in air (order of 160 picoseconds?), to circumvent the limitation on Transfiguring air, will only qualify as valid if they contain an experimental test of the ability to Transfigure air, together with a backup plan which is among the best available in case it is not possible to Transfigure air.
Pessimistic assumption: Plans which depend on Transfiguring antimatter will only qualify as valid if they contain an experimental test of the ability to Transfigure antimatter, together with a backup plan which is among the best available in case it is not possible to Transfigure antimatter.
Pessimistic assumption: Harry’s wand is not already touching a suitable object for Transfiguration. Neither partial Transfiguration nor extremely fast Transfiguration of extremely small volumes lift the restriction against Transfiguring air, dust specks or surface films would need to be specifically seen, the tip of the wand is not touching his skin, and the definition of “touching the wand” starts at the boundary of the wand material.