How is a Franco-Austrian combination more threatening to his Polish lands than the military and economic juggernaut of a Germany united under Prussia?
A Germany united under Prussia is no threat at all in the 1860s, as they have a common interest in keeping the Poles down. OTOH, during the Polish Revolt Austria showed some interest in a Polish kingdom under a Habsburg, while France was always sympathetic to Poland. Prussia is the only power on which Alexander can rely to see things his way. He wants a strong Prussia to keep both France and Austria in their place.
Anyway, what juggernaut? Germany's OTL population (excluding Austria) was only slightly more than France's in 1871 - 41 million against 36 million. And TTL France's population will be greater by at least a million if she doesn't lose Alsace-Lorraine. Per AJP Taylor [1] her 1870 Army estimated were £15 million against £9,6 million for Germany, while their steel production was about equal at 0.3 million tons. Germany didn't become an industrial superpower for another generation. There was no juggernaut in 1870.
[1] Struggle For Mastery In Europe, Introduction.