Ok, one thing I'm already happy about is that there does not seem to be just ONE right answer... I wouldn't want the players coming to me and say "the Emperor is just a jerk, making up rules". Of course, he's the Emperor, so he could if he would .

I'm going to answer's Andrew's questions, it seems they can help me clarify my thoughts on the matter, and let me see where the campaign might go from here on.

* what happens if Hrothwulf offends the emperor with his response? Or if a noble could claim to be enraged on the emperor's behalf?
The Emperor is already a bit offended, since he sent a messenger to Hrothwulf in the Giantdowns, inviting him to the capital to renew his father's oaths. Hrothwulf has never replied to the invitation. Through unofficial channels, he has already "warned" that an Anuirean army (led by one of Hrotwulf's childhood friends) may be necessary to "settle the issue".

* how many impoverished nobles, would-be counts, etc will seize on this request as an excuse to head north and stake a claim, offering vassalage of their own in exchange for land rights?
I didn't really think about this. I have my hands pretty full with all that is going on, so I'm not sure I want to introduce even more personalities in the campaign.

* how beneficial could imperial aid be? Perhaps this is opportunity not threat...
Hrothwulf would accept "help", but he does not mant to be a vassal. At the same time, the Emperor's court knows that the Emperor considers Hrothwulf a vassal, and he would lose his face if he left the matter unsolved.

* what vassalage ties are actually requested? Is this request mere vanity or is it serious?
It is sserious. The Emperor wants to use the Giantdowns as a buffer state to stage his future attacks to the Gorgon. And since Hrothwulf (and the other PCs) have been doing very well in the Giantdowns, he also expects the regents of the Giantdowns to be paying the salary of a lot of troops.

* what will the Rjurik realms say when they hear? What will they counter-offer?
The Giantdowns are a bit isolated from the other Rjurik reamls. The only "human" realm is the Realm o the White Witch, and Hrothwulf already has a few troubles with them (border skirmishes).

* what do the locals think, are they pleased, enraged? Where else could settlers come from?
The locals are mostly Rjurik, with a strong influence from the Emeral Spiral and the "rjurik way". So, they do NOT want to be annexed to the Anuirean Empire.

* will rivals of Hrothwulf consider this request "proof" that Hrothwulf is really Anuirean at heart?Definitely. And he knows that, and wants to avoid it.

* what does the regent/rival thereto of Cuiriene think about their 'rightful' lord abruptly coming back on the radar and inevitably casting doubt on the regent's position.
The present regent of Cuiriene will actually be more than happy to have Hrothwulf back. He was a good friend of Hrothwulf's father, and only wants the best for him.

* who inherits Cuiriene if Hrothwulf defaults - who actually granted the title, who could claim, etc, etc
I suppose it would stay with the present regent, but Hrothwulf doesn't really care one way or another.

* what does the Gorgon think of the new realm on his doorstep?
At the moment, the regions between Hrothwulf's realm and Gorgon are under the rule of the Warlock (from Warlock of the Stonecrowns). He will be the "end-game" boss that Hrothwulf and his companions will have to defeat to unite the Giantdowns. The Gorgon and the Warlock are now fighting each other over "family matters". Rest assured, the PCs have already made an enemy of the Gorgon.

* what do the goblin/etc raiders from the crown think of the new realm?
The heroes are exterminating goblins left and right. Of course the humanoids do not like the fact that civilization is coming their way.

* both father and son have now founded a significant domain from nothing - clearly the bloodline is strong - is Hrothwulf married? Could he trade up if so / be snaffled if not?
How do you mean "trade up/be snaffled"? (english is not my mother-tongue). Hrothwulf is not married. The Emperor had sent a noblewoman from Dhoesone to be married to him, but he refused. He does not believe in marriage for profit, and he's actually fallen in love with the daughter of the White Witch (a recurring villain in our campaign).

woah, this was fun to write... sometimes putting stuff in writing really helps ordering your thoughts!