Now, if Prussia fell to the constitutional side, Germany would be an instant great power. But I don't see how that can be done against the will of its military, its nobility, and its king.
Wikipedia said:
In
Berlin crowds of people gathered their demands culminating in an "address to the king".
King Frederick William IV, overwhelmed by this pressure, yielded verbally to all the demonstrators' demands, including parliamentary elections, a constitution, and freedom of the press. He even promised that "Prussia was to be merged forthwith into Germany."
However, on March 18, a large demonstration occurred and two shots fired by soldiers led to an escalation of tensions. Barricades were erected, fighting started, and blood flowed until troops were ordered to retreat a day later, leaving hundreds dead. Afterwards, Frederick William attempted to reassure the public that the reorganization of his government would proceed, and the king also approved the idea of arming the citizens. On March 21, he paraded through the streets of Berlin to the cemetery where the civil victims were buried, accompanied by some ministers and generals, all wearing the revolutionary tricolor of black, red, and gold which form today's
flag of Germany.
Provided this excerpt is correct, Frederick Wilhelm IV, having heard the fate of Louis-Phillipe in France, bows to the pressure of the revolutionaries and accepts their more moderate demands once it becomes apparent the army can no longer restore order. However, the Junkers are not permanently defeated at this occasion.
Wikipedia said:
General von Wrangel led the troops who recaptured Berlin for the old powers, and King
Frederick William IV of Prussia immediately rejoined the old forces.
If you can prevent a conservative resurgence and prevent the Prussian army from re-taking Berlin, Frederick Wilhelm would likely remain cowed and either abdicate, spawning even more chaos in the wake of his departure, or grudgingly agree to take the crown of a united Germany from the Frankfurt Parliament. I suppose this would not be too difficult to pursue (as the state of the Prussian army in 1848 was quite dismal; the Humiliation of Olmutz the most pertinent reminder of Prussia's neglect of her armies since the Napoleonic Era). The problem, though, lies in maintaining such an arrangement.
Provided things go as in OTL in Austria, a united Germany is likely doomed and a previous state of affairs restored (although Prussia will likely have to accept a harsher peace for supporting the liberal movement) under the jackboot of Austrian and Russian armies. However, if we assume things went differently in Austria and Italy (the Habsburgs, like in Berlin, fail to crush the demonstrations in Vienna and Lajos Kossuth is, thus, able to maintain an advantage, while violent revolutions in Italy spawn Venetian, Lombard, and Roman Republics that the Austrians are unable to control), it could buy some time for the Germans to get their house into order. Also, to keep the Russians off their backs, let's say the chaos in Austria spreads to Poland, triggering another uprising akin to the one in 1830. Provided the Poles conduct themselves competently and with the same fanaticism, it would take some time for the Russians to stamp it out, preventing military aid from reaching Austria, thus making the situation even worse for the conservatives.
By the time the Habsburgs and the Russians recover (provided Kossuth and the Maygars do not succeed), Germany should have at least gathered itself into somewhat of an organized fashion, with a partially united (but very divided) army. Of course, it would probably be without competent commanders for a time, the majority of experienced officers Junkers or other nobility and its conduct would fair poorly at first against a veteran Russian army (the Austrians could likely provide only a token effort at this point having been reduced after putting down so many different revolts) but, as the French proved against the Second Coalition, there's always a possibility to turn things around. Eventually battle-hardened commanders would move to the fore and, Germany's industrial potential harnessed, the new nation could very well repel Russia's assault and enter into an uneasy truce (Russia's army, too, was no longer the impressive force which had beaten Napoleon back to Paris).
As of this point, with a Republican France, Liberal Germany, and hand-full of new bastions for liberalism in Italy (including an enlarged Sardinia), the Concert of Europe lies entirely in shatters and the next following years are marked by a chaotic state of diplomatic affairs as new alliance blocs form. The Second Industrial Revolution and intense waves of Nationalism to come will ensure the next war will be an incredibly bloody affair.