where did you read East the Elbe was 'baltic' at Bronze Age - who knows that??? never red nor heard - a proto-satem or a not well evolved "between" I-E language, possible - it is not the same - the eastern Y-R1a of today East germany is not genuine in this land, it was surely carried there by High-Middle-Ages Slavs tribes which reached Hamburg surroundings + (it is proved by surnames inquiry) by modern Slavic emigrants - the Schleswig-Denmark region seems very well being the germanic cradle (with perhaps South Sweden), for we know at this date, not only the West of the Elbe (Laba)...
but if you have some proofs, I'm buyer!
good afternoon and good Sunday