I agree absolutely. It is the same as so many ICEV issues. Very few mechanics really understand how the thing actually works and rely entirely on a flowchart system of fault diagnosis that never really addresses the core issue, they just keep on replacing bits until it works.
If it is out of warranty, you might get more sense out of an EV-savvy engineer or even keen amateur. I bet it is something ludicrously simple, so simple that it gets overlooked by the flowchart analysis. Not that you will ever find out, even if it does come back to you, working.
When you say ‘too much power’ (I didn’t see the original post) do you mean too much voltage? I can see that that might be an issue as the T may well have a system to stop charging if the supply volts spike to high… If so, perhaps you should invest in one of those cheap (£10) watt-hour meters that plugs into the 13A supply socket and then the T’s plug plugs into it. Aside from telling you how much energy your T is using (useful when charging at a friend’s place so you can offer to re-emburse them - all 6p-worth) but it will also tell you the line voltage, power factor, current etc… It might just give you a bit more of an insight into what is going on if not ammunition to rebuff daft assertions by Renault’s useless ‘engineers’.