Let us have a more detailed scenario: the Austrian court misinterprets both Russia's intentions and France's strength, and joins France's side early (upon the occupation of Saarbruecken?). At this point, if my reading is correct, Prussia has 90.000 men in reserve precisely against an Austrian attack, while the forces on the French border may continue the offensive.
Of course, the Austrian intervention may cause a critical redeployment of Prussian units, causing the Prussian victories in August not to occur. At the same time, it would take some time for Russia to mobilize in Prussia's help. Austria would not want to fight Bavaria for political reasons, though obviously it is likely to have to (political repercussions in Bavaria will be interesting). So, wil Austria attack in Silesia? That puts Russia in easy position to strike at the main Austrian forces (Russia has the numbers to attack in Galicia at the same time, though the logistics may not be up to the task).
We may assume that Metz is still besieged and Sedan happens, both on schedule (it's by no means guaranteed). At this point, however, does France choose to resist, galvanized by Austrian support? And what is Italy going to do? An opportunistic attack on Rome may trigger war with both France and Austria. Irredentism was not a big factor in national politics yet, but historically, public opinion heavily sympathized with the French Republic against "autocratic" Prussia. This changes ITTL, but would Vienna keep fighting alongside a Republican French regime?
This may save Austria's ass: they seek peace after a few engagements, territorial losses (Galicia, Silesia, Salzburg, Vorarlberg), but the Empire stays united. Alternatively, Hungary sees the writing on the wall and declares independence (what's in it for them after all?). This forces Grossdeutschland without Pressburg.
Prussia is still pushing for A-L here: they can't back down if they are fighting still.
Too many variables.
It probably ends in a European congress.