It did not "integrate quite nicely", some of the people enjoyed the revolutionaries ideas and reform, some other resented the attack on Catholicism. You could say the area was a bit apathetic to nationalism than Lombardo-Venetia was but to think that would stay the case in later decades(I don´t know if nationalism is inevitable, but even if it isn´t they are not going to become neither Frenchmen nor loyal to whatever king rules France in mere decades). The whole of Rhineland even after the conquest was more connected to Germany than to France, like I said still apathetic to Nationalism to either side.
By Germany I meant the HRE.
By 1870, most of France still was speaking more local languages than standard French(that might have been true till modern broadcast), plus the area they did have were ruled for either half a millennium(Britanny,Occitania) or were small(French Flanders, Perpignan, Nice and maybe Alsace), Rhineland is a totally different thing.
A problem is the Austo-Prussian rivalry, if either of them is with France it would block any help for England coming from them, is it possible for them to resolve the question or have them just not fight for France? I mean I find weird that the Austrian just accept to lose such important piece of territory and fight for either of them, so I think neutrality would be chosen by the Hapsburg.
What would Russia and Spain do? I think the later would follow as OTL.
In this scenario both Austria and Prussia would be more scared of France than each other.