Short of a long and complicated TL involving a lot of unlikely but vaguely possible events whereby nationalist currents are averted against the odds, new states are discredited, and the only feasible option is slowly reverting national changes, this is pretty much ASB. I mean, we're talking a series of events whereby everyone in Central European history who should have gone on to be national heroes instead destroys their public image by making themselves seem manipulative and evil, thus discrediting their political ideas. But that is horrendously unlikely. The Congress of Vienna is just too late a POD for this to really work.
Even when it was on its last legs, the HRE was anachronistic and was looking like it was hanging over the precipice, waiting to be pushed off, as Napoleon eventually did in 1806. To save the HRE, you'd need a POD several decades before Napoleon (preferably butterflying his existence entirely) and you'd need absolute truckloads of Conversativism in Germany to make powerful figures unwilling to reduce the powers and authority of the Princes or increase the power of the Emperor. You could also do with pretty unlikely bucketloads of irredentism in the individual states, because mediatisation and Westphalian sovereignty ideas increasingly made the individual states have to fight for their existence in the face of pressure by the more powerful players in European politics to be annexed into the bigger states on their borders, and those bigger states to themselves be incorporated into the major states such as Hesse, Baden, etc. So long as pressure exists to roll up the smaller states of the HRE into large "Westphalia" type space-filling blocks in the way that the German Confederation forced all the mini-states to reform into 39 more sensible-sized ones, then the HRE is on the path to its destruction and a federal Germany is on the agenda.