Using extension leads with an occasional charger.
Don't do it. They tell you not to do it and there is a warning on the cable not to do it. I'm not going to tell you anything else other than don't do it!!!
..... However, if you are **** bent on doing just that for reasons of an emergency or whatever other reasonable justification you have, and I have not just dissuaded you, then here is a quick message and you should do at least this.
I strongly suspect the reason they do not want you going using an extension lead is to avoid overheating in the plugs. The plug connected to the box-o-tricks will, I suspect, have a thermocouple to shut the current off if there is an overheating problem. You will probably not have such a thermal protection at the other end.
The other possibility is that the extra extension adds inductance to the line. I doubt it would have much of an effect, but in theory if there is enough inductance added then throwing a switch and shutting off the current quickly could induce extremely high voltages in the line.
If you are using an extension lead, then at least test you have good plugs and sockets in the following simple way - after two hours of charging, power off (best to switch off the charging by removing the plug from the car, which stops the current by inhibiting the car drawing current, rather than switching off the mains socket) and unplug the charging lead 3-pin plug. Grab the live pin and pinch it between thumb and forefinger. It should be 30~35 C. If it is hotter than that (which you will know because you'll think 'gee, that's HOT', or '**** I've burnt my finger!!!', instead of, 'hmm, OK, it's a bit warm) then the socket and cable to it is not fit for the job. (That's true of a wall-socket as much as one in an extension lead assembly.)
Then do the same with the extension plug into the wall socket. If that one is 'HOT' then either the lead is not of sufficient quality or core gauge, or, again, the house socket and cable isn't up to much to be good enough, or both. You might want to be more cautious about grabbing this, as this will be more likely to be hot enough to burn your fingers. (Better you find out with burnt fingers, than burnt house!)
You could always use an IR thermometer too, if you have one.
As a general rule, you shouldn't be using plugs and sockets with pins that get too hot for you to comfortably pinch and hold on to, whether it is charging an EV or any other appliance. The problem is that as they get hot, the resistance increases and they can enter 'thermal runaway' and that's when you get unintended 'thermal events'.
So, if you feel compelled to use an extension lead, or in some way it is an emergency, check the pin temperatures of both of the 3-pin plugs involved after a while charging.