Hm, Roon. I admit not knowing much about him. But by the time of the PoD the course had already been set: Bismarck was adamant about supressing the parliamentary opposition, and also had started to assiociate with the nationalist movement.
Roon will have no other chance than to try to turn attention away from domestic problems by having diplomatic and military victories - same as Bismarck tried. Thus, Prussia (and hence Austria) might enter the war as IOTL, and hence it would also go more or less as per IOTL.
But afterwards... Bismarck had a specifically anti-Austrian bent in his conception of foreign policy. Roon was a political charge of Bismarkc, more or less, but I dont know wether he had that, too. He was foremost a military man, so who knows wether he had any grand cocneptions in foreign policy at all! So, might well be Prussia does not provoke Austria and the German Confederation into the (OTL) German War of 1866. However, the situation in Schleswig-Holstein is simply unstable: Prussian Schleswig, Prussian Lauenburg, Austrian Holstein (an exclave with absolutely no value for it!) and Prussian marching rights through Holstein. Its a receipt for cofnlict, and IOTL: its just that Bismarck sought, after all. However, even sans Bismarck the Duchies will most likely be split between Prussia and Austria, and, yes, its unstable.
As it is, Austria tried to reform the Ger,man Cofnederation anyways, at that time, so as part of that it might turn over Holstein to Augustenburg as independant Duchy, and that could well provoke Prussia... though that would at elast delay the war, and in 1867 theres the Luxemburg Crisis. However, a longer standoff between Prussia nor Austria means neither side can afford alienating the nationalist elements in Germany, espeicalyl after Bismarkc already had begun to try to associate with them. Hence, both Austria's and Prussiia's reatcion to that could be quite strong, and with both great powers agreeing, the German Confederation could act as a whole, and even try to provoke France into war over the whole affair....