Well … a couple of points:
(1) The fact that the decree was from Pius X does not mean that it is “religious doctrine.” What it does mean is that Pius X allowed the custom to continue.
(2) Now, whether the Oriental Congregation “unlawfully” overruled a Papal decree is debatable. The Oriental Congregation did not upend the decree in respect to Kottayam, which was the point of the decree in the first place, so technically they did not act unlawfully. And I think it’s worth noting that the Oriental Congregation, as do they all, functions with the authority of the Pontiff unless the Pontiff personally demands or countermands a particular action, and I haven’t heard of that having happened in this case.
OTOH, what we do have is a case where the rule (or “law” as it were) is being applied unequally. By that I mean there is now one set of rules for Kottayam and another for the diaspora. While that is not technically unlawful, it seems to me that it is quite unfair. The problem is that there is now a diaspora outside the Archdiocese of Kottayam and indeed outside India. Pius X did not address that because it really wasn’t an issue at the time, and unless or until it is addressed, I don’t see a way out.
It is, of course, possible that Pius X’s decree could be amended or superseded, or even nullified, by a successor, but I tend to think that if the latter happens, whether directly or indirectly through an amendment or a new decree, it will result in a Knanaya exodus. And I think that was the reason for Pius X’s decree in the first place.