Sounds interesting but...
1: Does it get Hannover? In what way and how do the British react?
The crowns of Britain and Hannover got separated in 1831, and supporting the dynastic interests of the Saxon-Coburg-Gotha in Germany is no major priority of the British ruling class, they would be apathetic to the fate of Hannover like in OTL 1866.
2: Why doesn't Russia or indeed Prussia offer to help Austria against its dissidents united as they were at this time against revolutions and major disruptions of the balance of power?
To be successful, a Prussian-led Greater German unification in 1848 requires two things: a PoD that paralyzes Russia during the year (not any difficult, this is the year of revolutions) and a Prussian King with brains, who realizes that siding with the liberal-national movement is the future of his dynasty and his country (a lot of PoDs can give him a different personality). Neither PoDs are too difficult to accomplish.
3: Are not the Austrians going to be rather rebellious subjects of these Prussian upstarts?
The Prussians saved them from the collapse of the Habsburg Empire, and Prussia gets the helm of German unification, so German nationalism makes the Austrians loyal, as much as the Bavarians were OTL.
4: Are not Britain, France, Russia going to declare war on this massively aggressively and expansionistic Prussian state?
France is paralyzed by its own revolution for about a year and half, until it re-establishes a stable government. Don't count on it. Indeed a PoD that paralyzes Russia is necessary. Britain can't do much in central Europe without the support of both and was apathetic or sympathetic towards German and Italian unifications. After 1849, when the new German, Italian, and Hungarian states stabilize, undoing the change would take a general European war, and neither Britain nor Russia have an overwhelming interest to start one at the moment. Greater Germany, Italy, Hungary, France, Russia, Turkey, Spain, and Britain still make for an meanginful blance of power in Europe, even if power shifted to the CP bloc. Russia has no meaningful geopolitical quarrel with said bloc either if the CPs support or are neutral with Russian expansion in the Balkans at the expense of the Ottomans. Even Russian legitimist feelings are going to make themselves content with the collapse of the Habsburg if it is a fait accompli and they were friendly with Prussia to begin with. France is definitely unhappy but starting a war with Germany, Italy, and Hungary without allies and a plausible casus belli is a recipe for disaster.
5: Are the Hungarians going to be able to organise themselves in time to contain the inevitable revolts amongst the rumanians, croats and perhaps more importantly the Italians. I can see them loosing a lot of ground very quickly.
Italy surely gets all its irredentist claims (Trento, Kustenland, Dalmatia) out of the Habsburg collapse. But just like the Germans after they get Austria and Bohemia-Moravia, this turns them into supporters of the Hungarians. A viable Greater Hungary kingdom becomes a bulwark against Pan-Slavism, which would threaten Italian and German possessions. With German and Italian support, I can totally see the Hungarians (which were much more organized than their own Slovak and Romanian minorities) organize themselves in time to stabilize their kingdom, esp. because their uprising is a major reason of the Habsburg collapse. Croats were rather organized, too, which means they probably have to be conceded some meaningful federal autonomy within the Kingdom of Hungary. This however would not be out of question for the Magyar ruling class, Croats traditionally had some degree of autonomy, Budapest is just adamanet at keeping full control over Slovakia and Transylvania.
6: Who will be the Hungarian Emperor?
Lots of possibilities: a suitably liberal and cooperative Habsburg scion, an Hohenzollern-Sigmarigen, some member of a minor German royal line.