I was surprised to see that WG2 has accepted a proposal made by the US National Body to use CGJ to distinguish between Umlaut and Tréma in German bibliographic data.
You raise some interesting questions. However, note that the purpose of CGJ is intended for sorting related distinctions, which are at issue here. This is different from variation selectors which are intended to be used for displayed variations.
Since sorting is *language* specific and needs language specific sorting tables, the use of CGJ is also *language* specific.
Therefore, the combinations for which CGJ would be used in the type of bibliographic records are a small an fixed set. For display, CGJ is supposed to be ignored.
Furthermore, the distinction between umlaut and is not something that's generally carried, but limited to specialized data such as bibliographic records.
All these factors need to be taken into account when comparing this to a general solution that's *script* specific and is not limited in its domain of application. You might find that some of the concerns raised in the context of the other discussion no longer apply.
A./
