I posted earlier in this thread asking what references to give a dealer who was skeptical of the problem.
I accidentally found the best solution in the end was just to ring up another dealer, which I should have tried sooner. The conversation went like:
"oh, and my bikes got the J00 ECU which has the Suzuki advisory out for it and an updated J02 unit available, are you able to sort that at the same time?"
"yep, no problem"
[...me absolutely speechless... no argument, nothing, dealer fully aware of problem]
About 2 days to order and then fit on the spot.
I'd got pretty miserable about the bike prior to this and was getting nowhere with the other dealer. New dealer fixed everything including the ECU.
For me I found the TRE fixed the stalling and some behaviour problems, but now replacing the ECU just felt like someone had replaced the engine with something a whole lot stronger. Feels strong low down and is now fine in slow traffic whereas it was a right pain initially and was still a bit of a grump even with the TRE. Also under hard acceleration it just feels like you're riding a big smooth acceleration curve. Feels to me like you noticably don't have to rev it as much to pull away fast. I think someone else mentioned it no longer felt like it was accelerating out of a hole and I didn't really get what they were describing until riding with the replacement ECU and realising what had been missing.
Anyway, short version: if a dealer is being a pain, try ringing another one, it might save you a lot of time.