Except that Machiavel doesn't have the authority to recognize any state as legitimate successor.
In RL law, false. Thus insofar as BM is an analog for RL, also false. Machiavel is both a government official capable of creating estoppel rights in his office as Mootgram, but, maybe more importantly, he is a sitting monarch.
The fact that his public proclamation was erroneous does not change the fact that it happened. Legally, Machiavel, and any office he held, has recognized Phantaria: the Monarchy of D'Hara and the office of the Mootgram, to wit.
Only the elders can. As as the elders haven't voted on anyone being that successor (because, despite what Phantaria said at the start, they never did make that formal request to adhere), there are no legitimate successors. Not Terran, not Phantaria, not Saffalore. The Old Republic is dead, and there are no inheritors.
There are always, always, always inheritors. A place once it ceases to be Terra Nullius can never return to that state.
As for Terran, you seriously expected us to recognize it?
Actually, yes.
It had, what, four nobles left? A single region, starving, under siege? And you expected what, that we come and defend it?
You signed the gosh durn treaty saying you would. This is precisely the kind of incident the Treaty was envisioned to prevent. Hireshmont will sit back and giggle when D'Hara has a secession crisis someday and everybody says, "Well, that's really not my problem. There is no real D'Hara."
The majority of the nobles of the old republic were opposed to Hireshmont and his little gang in the Chateau.
The political legitimacy of a government has no bearing upon the legal legitimacy of a state.
There was no compelling reason to alienate the rest of old Terran.
Right. That's why you're now at war with Saffalore, and on the brink of war with Phantaria: because you want to avoid alienating the parts of Terran.
Furthermore, Terran never made a formal request to get it's full membership back. You just went "Hey, what the heck?", then gave up.
False.
And the treaty governs how the signing parties will act towards the cited land.
No. The treaty governs how the signing parties will act towards one another. You can't sign a treaty with land.
It doesn't say anything about who holds it, or under what circumstances the obligations to the cited lands cease. Chesney is on D'Hara's and Barca's doorstep, it remains of special interest regardless of what happened to the Old Republic.
The obligations obviously cease when the Treaty which governs the relationships between the sovereign states holding those lands which are governed by the treaty are terminated. This is seriously like not even law 101, this is like the stuff you learn before you take real law classes so you don't make a fool of yourself.
