Oh, I completely agree that in the beginning the firmware was a hot mess and yes, I had issues with it as well..
Nobody in the beginning got an SD card with the firmware on it - we all had to go get it from the google drive, and flash it ourselves - so you are not a unique case there. Anyone that did get an SD card was the unique case.
But, let me tell you this..
My very first printer was a used Solidoodle 4. The stock hotend on that had the thermistor taped to the outside of the heatblock - that was how they were made. It was also PEEK lined, so high temps were off limits
You want to talk about temperature differences?? how about around 40-45 degrees off?
I STILL never used a thermometer to figure it out.
Knowing that "most" people were printing PLA at around 200C, and ABS around 235-240C - I thought there was something seriously wrong when I was burning PLA at 200 and could not even get it up to 235 for ABS, as it would shut down at 225 with the "thermal protection" in the firmware. I learned, however, that a lot depends on how the hotend is set up, where/how the thermistor is placed, and what type of thermistor it is - some are more accurate than others, and placement makes a HUGE difference. Also, having the correct temp table setting selected in the firmware makes a difference.
I used temp calibrations to figure out optimal printing temps for that machine... for PLA it was set to about 160-165C - for ABS it was more like 190-195C - depending on colors being used - some printed best hotter, some cooler...
When I changed out the hotend for an E3D (because I eventually wanted to try printing Nylon and other high temp filaments), I had to learn how to flash the firmware to make the necessary changes, and had to recalibrate temperatures all over again. Not once did I need a thermometer to do any of this.
Every machine will be slightly different, every hotend will be slightly different - or in some cases, drastically different - as my SD4 was.
And to top that off, every filament will be slightly different in its temp requirements. It is part of the calibration process to figure out the optimal temps for YOUR machine and your filament, specifically.
While I agree that 20 degrees off on the hotend is a bit drastic, it is not that uncommon - especially in a kit printer.
And, while a DVM makes the job of setting the stepper driver voltages a bit easier, it is also not a complete necessity. It can be done without one if one needs to do so.
Please do not take this the wrong way, I applaud your determination to figure out what was going on with your machine, and the extreme lengths you went to to do so. All I am saying is that one does not necessarily "need" to go to those lengths to make things work. A few temp calibration towers would have told you the same thing - that your temps were off by around 20 degrees. Adjust your temp settings accordingly and you can still print successfully.
I used the stock hotend on my SD4 for several months - quite successfully, btw - before I invested in the E3D. I wanted to make sure I could actually make the thing work before making that investment.
That same SD4, with the original E3D v6 hotend I bought years ago (shortly after the v6 was first introduced), is now my "workhorse" printer producing prototypes and regular parts runs for other people.