Ok fair enough, but Costco HAs aren’t locked, so in “theory” any audiologist’s clinic could provide programming, but yes it’s always much more expensive, that’s sad really, but the same everywhere.
But getting back to your questions, I’d pay the €1920 and work a deal out to keep the Widex as back ups, because as you say, there’s no value in selling or even just “giving” them to your audiologist, they’ll only throw them away anyway, it’s only €100 difference between lifetime warranty and 2years, Nana could make a century!
I wouldn’t worry about the different platforms at all.
Good luck.