No it isn't, your "mathematical analysis" fail to take into account a huge number of variables, such as spell options, terrain variables, players attentions, DM decisions, not to mention the dice rolls.
Obfuscate all you want. An unremarkable 1st level monster can gank a low-hp 1at-level PC with a lucky shot or a pair of fairly ordinary attacks.
Do you really think your 'huge number of variables' will result in no low-hp PC ever getting critically hit, or hit twice without being healed in-between?
Can 1st level characters die in 5e? Yes.
Can it happen to new players? Yes.
Can an inexperienced DM "accidentally" kill low level characters? Yes again!
Can 1st level characters die instantly in 5e in a normal encounter-guidelines combat, in the surprise round? Yes. In a single round or to a single crit? Yes.
And, since you like annecdotes, yes, I've seen those things happen in the playtest. A 1st level character killed in the first round of the combat of the first session of an event; characters killed because they were up and fighting at a few hps, and a high damage roll was enough for instant death. Always at 1st or 2nd level.
Does all that hold at 'higher' levels, even as low as 3rd? Not really.
If you're running for 3rd level PCs, you can pretty easily design an 'easy' combat to introduce the players to the game with no real risk that someone'll get instantly killed unless there's just an atrocious string of bad rolls and bad decisions.
Again, since you prefer anecdotes to verifiable fact, I never saw a 3rd+ level playtest game result in similar sorts of character deaths. Indeed, in the Encounters season I ran at 6th level using the playtest rules, even though there were some battles meant to be difficult, there was not a single character killed, nor even dropped.
For 1st level, not so much. An encounter weak enough to minimize the chance of a sudden, pointless character death, is unlikely to provide much challenge or interest - or much chance of a less sudden character death that the player might at least draw some lesson from.
That's not a fatal flaw of the game or anything, but it is worth considering - especially when tossing together introductory or casual-play adventures.