Well, there's two potential solutions - the language is of Corded Ware origin, or Indo-European languages (ignoring Anatolian languages, which is a whole different debate) are ultimately not from the Steppe. I lean towards the former solution heavily. Satemisation would happen later on, on the Steppe, potentially as
here (von Bradke's hypothesis; controversial).
All Western IE languages derive from U152 and U106 groups - the only two Central European L51 subclades. I do not think this is a coincidence - this is the meeting point between BB and CWC.
And I'm not sure what you mean when talking about Cucuteni. As regarding the Balkans, I see it as the main breeding ground of R1b folk since the
Epigravettian period (neatly explaining things like Villabruna being R1b - Epigravettian is in pink by the way, whereas Solutrean is in red). I think M269 originated in the Balkans, and that it spread into the Middle East with the very first Chalcolithic cultures (Vinca, Halaf etc.). I think that hierarchies with a farmer peasant class and a metallurgical/pastoral elite class were present during this expansion. L23 would form somewhere in West Asia, and I think, from somewhere in West Asia, pre-Z2103 and pre-L51 split, with what would become L51 going by sea to Southern Iberia and what would become Z2103 continuing on in West Asia (both in search for metals). Z2103 would later go up into the Steppe (Yamnaya etc. - potentially for their flocks), whereas L51 would later expand across Europe as part of the Beaker complex.