Wasn't Quirrell's whole assassination plot doomed to failure?

Question

This is part two of my query about Quirrell's murdering schemes in Harry Potter and the Philosopher's Stone. The first question was: Why didn't Quirrell try to kill Ron and Hermione?

I've accepted the idea that Quirrell chose the Quidditch game to try and kill Harry because it would've been easy to claim that Harry falling off his broom was a terrible accident. From Morri's answer:

Harry's broom in his first year, the Nimbus 2000, was a racing broom which (with its quick acceleration) might require skills beyond an average first-year's capabilities. It could be that Quirrell hoped people would think that Harry fell to his death due to his inexperience.

The original plan was probably make it appear an unfortunate Quidditch accident - a seeker too young to participate lost control of his faulty broom, and alas! died or suffered a brain injury.

However, wouldn't Quirrell have seen the numerous downsides to his plan for killing Harry during Quidditch? Namely:

• At the Quidditch game there are plenty of people who can save Harry or come to his aid by countering Quirrell's magic. Snape ends up saving Harry and he wasn't even aware of Quirrell's intentions beforehand; he just responded on the spur of the moment and was still able to save Harry's life. Any of the teachers could have done this.
• When Harry fell, he could've been caught by one of Harry's teammates, who were ready to catch him if required.

  The whole crowd were on their feet, watching, terrified, as the Weasleys flew up to try and pull Harry safely on to one of their brooms, but it was no good - every time they got near him, the broom would jump higher still. They dropped lower and circled beneath him, obviously hoping to catch him if he fell.
  _{(Philosopher's Stone, Chapter 11, Quidditch).}

• There's a spell (called a Cushioning Charm) which makes large falls non-fatal:

  [Dumbledore] ran onto the pitch as you fell, waved his wand, and you sort of slowed down before you hit the ground.
  _{(Prisoner of Azkaban, Chapter 9, Grim Defeat).}

  If Dumbledore can save Harry's life this way during a Quidditch match in his third year then presumably he could've done the same in his first. It's not a spell which is unique to Dumbledore either; Hermione uses the same spell for the same purpose in Deathly Hallows.

• By attacking Harry in public Quirrell makes it more likely that someone may, however inadvertently, catch him in the act.
• The death of Harry Potter would not have passed under the radar. Harry was an A-list celebrity in the wizarding world. His very public death would've surely invited a lot of press intrusion and a Ministry investigation - all of which makes Quirrell's ultimate goal (stealing the Philosopher's Stone discretely without anyone noticing) more unattainable.

Why then did Quirrell settle on the Quidditch plan? Couldn't he have found some other, more subtle way to silence Harry and make it look like an accident?

Answer 1

At the Quidditch game there are plenty of people who can save Harry or come to his aid by countering Quirrell's magic. Snape ends up saving Harry and he wasn't even aware of Quirrell's intentions beforehand; he just responded on the spur of the moment and was still able to save Harry's life. Any of the teachers could have done this.

The point is that no teacher besides Snape did it. The first quote in your first question provides evidence that no other teacher suspected anything, because otherwise we would've seen some effort to provide protection at the next match, beside just letting Snape referee. The other teachers didn't suspect anything at the time, so they were caught by surprise. Not the best start for an appropriate reaction.

When Harry fell, he could've been caught by one of Harry's teammates, who were ready to catch him if required. [...] There's a spell (called a Cushioning Charm) which makes large falls non-fatal:

Quirrell states:

"Another few seconds and I'd have got you off that broom. I'd have managed it before then if Snape hadn't been muttering a counter-curse, trying to save you."

I guess his original plan was to have Harry off the broom before anyone realised something unusual was happening. During the match, I would expect most eyes to be fixed on the action, not on a Seeker who was just looking around, so that's not an unreasonable idea. The crowd would be looking at the Chasers, Keepers and Beaters, the players would be focused on the game (except the Seekers, who would focus on looking for the Snitch) and the referee would be focused on the most likely spots for fouls. It was an optimal moment to catch everyone off-guard. The presence of people who won't react is irrelevant to the success of the plan. Snape's reaction (buying Harry time and letting everyone realise that something was unusual) ruined that, but I doubt that Quirrell expected that course of events.

By attacking Harry in public Quirrell makes it more likely that someone may, however inadvertently, catch him in the act.

It would've been hard for Quirrell to attack Harry in private. There are other students around in the lessons, and he would surely have to answer to Dumbledore if a student got killed during his class.

Outside of the DADA lessons, Harry was mostly in other lessons, in Gryffindor Tower or with other students like Ron and Hermione. I doubt there was ever a good chance for Quirrell to kill Harry in private.

Additionally, he might have refrained from using the Killing Curse because he knew Harry already survived one Killing Curse and neither he nor Voldemort seemed to know why. Arranging an 'accident' in a scenario where it's plausible for Harry to have an accident looks like a good idea to me.

The death of Harry Potter would not have passed under the radar. Harry was an A-list celebrity in the wizarding world. His very public death would've surely invited a lot of press intrusion and a Ministry investigation - all of which makes Quirrell's ultimate goal (stealing the Philosopher's Stone discretely without anyone noticing) more unattainable.

True, but that holds for every possible assassination of Harry - and even for most other ways to silence him. For example, "Harry Potter in St. Mungo's for sudden loss of his memory" would've raised the same problems.

Why then did Quirrell settle on the Quidditch plan? Couldn't he have found some other, more subtle way to silence Harry and make it look like an accident?

I think Voldemort wanting revenge played a great part. Silencing Harry wasn't necessary, because he suspected the wrong person and nobody took him seriously anyway. Quirrell could've done nothing, without endangering his ambitions for the Stone. The biggest threat to him was Snape; Harry and his friends were very unlikely to prevent him from getting it. His actions only served to make them more likely to disturb him. The reasonable choice of action was to ignore the first-years and do something useful with his time.