What a strange one Kitz!
You might have put the text exchange in to show us just when you realised what might have happened. But as soon as I saw the start of the exchange, I could see what was coming.
I have seen something that, in hindsight, was vaguely similar once, but not in the broadband sphere. An unexpected swap of hardware causing havoc.
As for *why*, I guess Bob hits it with the TR-069 reference. From what we know now, I'd say the TR-069 database needs to include entries for both devices (and the account they belong to) and entries for lines that qualify for auto-configuration. I'm guessing, but perhaps the device has always been there, associated with your account, but your daughter's old line wasn't listed. Once swapped to a new, qualified, line, then TR-069 took over.
Obviously this overrode the PPP authentication data, so I wonder if TR-069, operationally, is a BTW/Openreach matter, rather than an ISP - which, by definition, only gets involved after the PPP authentication stage.
As for the foot, I recall a phase of doctor-induced gout in my ankle that made it impossible to walk or drive. I can imagine both the agony and how it makes day-to-day living impossible. Owww.