As someone who has been paid to fix things for over 50 years, I know to confirm the item in question before replacing it. Confirm that it is the problem.
do you concede the point that disconnecting the 12 volt battery erases the trouble codes?
do you concede the point that if the 12 volt battery were the problem, that simply attaching a booster with jumper cables or a battery charger would eliminate the problem if it were the 12 volt? there by ruling in or out a low battery supply. A quick simple confirmation that for whatever reason, it wasn't receiving enough 12 volt current, but no one suggest this simple test. It still doesn't mean the battery is at fault, but at least you know where to start looking. I find it funny no one suggests that!
I can cause a "no start" (no ready) by simply trying to put it in gear before it has finished its self check.
There are many ways the system can be confused just by not letting it finish, some of those can trip an interlock which will prevent it going to ready, and by removing the 12 volt supply, you reset, whether if it is to install a new battery or just to re attach the old one, makes no difference.
If it will go though the self test and will move out of park, but not into gear, it is NOT the 12 volt, it has pulled in the main contactor and it is something else that is not allowing it into ready mode.
The main loads on the 12 volt are the main contactor, park pawl motor and only once in ready mode, the steering and brake assist, although these don't come into play right away.
It is very evident to me that many have not bothered to download and read the service manuals that are available for free. These very same people seam to feel they know more than those who have read the manual, although they can't come up with documented proof of what they say. (not pointing at you, but in general)
Most of the situations where "battery replacement" is being recommended are classic signs of an interlock being tripped and preventing it from going into ready mode. If it was a one off com glitch, re-setting (erasing the code) would allow the system to go to ready. You can do that by erasing the code with the OBD 2 scanner or by removing the 12 volt, either way, once the code is gone, the start will happen. If however it is a more serious problem, like HV isolation leak, then the code will not clear, or will return on start, and the interlock will remain, no matter how many 12 volt batteries you put in.
If you treat the symptom but not the root cause, it will keep coming back.
The 12 volt battery in the Leaf has one of the easiest duties there is, no glow plugs followed by cranking a diesel, no being shoved up against a hot engine or radiator and no high current loads.
To suggest that Nissan engineers don't know how to charge a lead acid battery, but people here know better is laughable. The service the battery is in is different than most here are familiar with, so don't look to what you old car did and say that is the best way to do it on a Leaf.
We have people at both ends of the spectrum, those that say the Leaf charging system and battery are useless and need constant changing, and those that hook 1000 watt inverters to the exact same and claim no problems.
Except in very rare occasions, before I change a part, it is confirmed beyond a shadow of a doubt that it is the CAUSE of the problem.
The list of codes that will trip a fail-safe or interlock is a long one, and a little "noise" on the com bus can cause an intermittent or one off problem, that can lock off a function.
I currently have three of the batteries that the Leaf uses in my stable, They vary in age but have yet to have one fail. Two of them are in much more demanding applications than the Leaf.
Over the years I have worked on all kinds of equipment from industrial, aircraft, power generation, motor vehicles, just about anything, and no one like someone who replaces parts and doesn't fix the problem. Do too much of it and you'll be out of a job in the better shops. Diagnoses is difficult and can be time consuming, and often isn't compensated well in commercial shops, but that is no excuse for not doing it and just throwing parts at the problem.
I'm sure I will rub some the wrong way, but I will not agree just to get along. Most cases I'll back up a claim with the documentation, in cases of how things work. In cases of helping people, it is more difficult, they are with the car and we are limited by what they see and relate. We just had one where there are differing ideas on what it means to "turn on" or "start".