I do think it adds a lot of complexity to Array transformers (from/to), would it be possible to add some kind of abstraction to separate the pure mapping part & the identifier/existing values handling in separate class / traits or whatever ?

Yeah but all of this is internal and it's only on implementation, i don't want to over abstract thing, as it may changes and other bugs may appear, we can always change it latter since it's internal, i first want feedback on using this feature before abstracting the code and public api is minimal on this (so it would not impact anything)