I hope you don't mind me asking, but how has this developed over the last week?
Well...
Apparently Zen are very keen to resolve. Someone from the tech complaint team phoned me to offer huge apologies. Noted I'd already made the decision to leave, but asked if I would give them a day to find out what has happened. The ridiculousness of the original case probably helped. I decided I had nothing to lose, and I was interested how they could so monumentally screw it up repeatedly.
Long story short, they believe the issue is with Openreach or at least they need their help/engagement to fix it. Happening on very few lines which makes it difficult. No suggestion they would subject me to continued horror trying to resolve the GEA issue, but keen to get me back on wholesale and actually stop a future migration. There was a flag on my account, but if the flag is not noticed and the line not manually excluded from scope each time a migration script is run, then it will be migrated. However, they have confirmed with the NOC that there is a backend manual change that can be made on the actual system/line itself so even if the script is run, the line will report back 'nope, I'm not doing that until the year 2033' (or insert date here). Sounds like this is not something that is routinely done.
I'm still clear to leave if it all goes wrong, and they'll credit any term fees if I do, but given I have spoken to the exact same actual human several.times not reading from a script, who I think is genuinely horrified, and who has kept me up to speed honestly, and openly throughout, I have decided to chance it. He is going to touch base on the day of migration (next Friday - stuck with OR standard 10 day turnaround) check all is good, then ensure the backend change is made and confirm back.
Every now and then I think I'm mad, then I think that the GEA issue is the only problem I have with Zen. In selecting an alternative there has been a large compromise for all - cost if I want to be able to talk to someone that has half a clue what they're doing (AA), A naff call centre, or no static IP, or no IPv6 if I want to limit cost to similar to what I am paying. Annoyingly, GEA aside, Zen does seem to be the sweet spot for me.
I'll report back in 4-6 weeks