Allow me to put 2 cents in here... I am a general dentist/woodworker...I do root canals. Your discussions are all valid, to a degree, but I think you've failed to "hit the nail on the head".
What differentiates the different scenarios has something to do with the person and their resistance to infection... it has something to do with the tooth and how many canals and how easy or difficult they are to thoroughly clean and it has MOSTLY to do with the bacteria that are causing the painful infection. While there are some scenarios that are difficult to explain, the great majority of cases (statistically in the high 90 percents) are straight forward, with zero or only minor painful episodes AFTER the infection is under control.
The longer the person endures their discomfort/pain before initiating treatment, the more difficult it is for anesthetic to completely numb the tooth. We refer to that as the 'hot tooth' syndrome... for no known reason, everything can seem thoroughly numb, but when the treatment is started, there is still intense pain... more anesthetic than usual is required and/or it takes a long time to "soak" in and take effect. Again, over 95% of the treatments are painless from beginning to end... with this exception... and the patient almost always plays a role in creating the exception, although it is the bacterial infection that is to blame.
In addition, the longer someone waits for treatment, the more the bacteria establish the tooth and the surrounding bone as their 'home'... so the treatment of the tooth and its infected nerve is sometimes not enough and antibiotics are needed... and the longer the infection sat festering (sometimes painlessly) the stronger the infection when it finally causes enough pain to go to the dentist. We don't exactly know why an infection can sit quietly for a long time and then suddenly flare up like gangbusters, but it often relates either to the type of bacteria and/or a decrease in the person's ability to fight off the bacteria. (So being overworked, or not getting enough sleep, or being very emotionally stressed, or fighting off another infection like a cold or flu or whatever, seems to allow a small or quiet infection to snowball.) Again, it is the bacteria that are to blame and they are not all alike... since antibiotics are not usually needed, no antibiotics are prescribed... and the result after the initial root canal visit usually is 1-3 days of discomfort (not pain) that subsides to nothing. The exception is treated with antibiotics, but again, there are differences... the tried and true antibiotic of choice is penicillin for dental infections - and it is effective, again in the
90% range... but if/when penicillin doesnt' seem to be effective, which is sometimes related to how long the bacteria have been active, etc., the more chance there is that a newer/stronger/significantly more expensive antibiotic will be needed. During that time, when the right antibiotic is determined and it actually starts working, the patient is often in pain... but the culprit is not the person, the tooth, the "root canal treatment" or the dentist, but the bacteria and the infection they produce.
Another variation on the theme is that bacterial infections that aren't overly aggressive may create pus that finds its way out through an opening in the gums... in other words, it ends up leaking pus into the mouth through the equivalent of a pimple. Because of this, the person may notice a bad taste but doesn't usually suffer much pain. Other infections create pus rapidly and, if they have no quick way out, the pus can leak into parts of the face and you can wake up with part of your face swollen. Often, these infections were known about for weeks or even months as discomfort came and went, but there was never much pain until one day when it all changed... again, the pain and swelling, etc. is from the infection, not the treatment. And often the person suffers extreme pain if the pus builds up very rapidly and has no place to go (yet)... the pressure buildup causes the pain... and sometimes the bacteria create gases which increase the pain/pressure... until the pus finds someplace to let out... which is rarely like a small pimple in these cases... and the face gets quite swollen... and as the swelling makes them look worse, the pain lessens because there is less pressure in a confined space. Again, the villain is the bacteria, not the treatment... and if getting numb is not simple, it is not the dentist who is to blame, it is consequence of severe infection.
Bottom line is still: save the tooth if it can be saved... root canal treatment is the method used to save the tooth... over 95% of the cases are straightforward and close to painless... and prevention or early treatment usually pays off with fewer complications.
It is my belief that the painless (or nearly painless) root canal treatments cause no comments... but the rare painful cases cause the story to be repeated over and over so that the reputation of root canal treatment is that it is a painful procedure... but it just ain't so.
I think this was a pretty complete general consultation on the issue and I said all I have to say... so I reserve the right to not hold online consultations about individual situations... if you post questions or email me, I may or may not reply. I hope you understand.