A CP victory would make even it to Maria-Adélaide and esp to whatever Minister might serve her at the end of the wsr clear, that whatever international franco.british backed 'guarantee' therer migth be uttered with giant as the german empire at your dorst... sitting on your lap they aren't worth a drop of ink.
They would be happy to be incorporated into the confederation the German 2nd Empire was without any conditions (if v.Eyschen's still around some langue and school issue mitgh paly some side role) at all. ... and NOT being outright annexed into Prussia what the other german states (Saxony, Bavaria, Württembergia, Badenia) would strongly oppose anyway.
However. To make it look somewhat more 'acceptable' I could imagine that the Briey-Longwy industrial region - or only the Longwy region with the Brieyre-gion to Prussia or under some kind of temporary Saar-statute like deal -might be added to Luxemburg as well as some of it former parts that were given to Belgium in 1815 and 1839 (region of Arlon and Virton maybe ?).
So :
no need to cede prussian territory at all.
..., King of Prussia was his second job after all.
Sry but ... King of Prussia was his FIRST job, Kaiser and head of the conferadtion the german empire constitutionally was came always onlysecond - as already with grampa Wilhelm I.