Thanks for sharing your findings, really interesting!
His death certificate has name Marcus Samuel Loewenthal, so it makes sense that this was his 'official' name. It's certainly an amazing coincidence if this is not his family.
There's a possible husband for Amalia Löwenthal, Nathan Grünsberg. One of their sons could have been Max who emigrated to the US as Max Greensberg.
"New York, New York City Marriage Records, 1829-1938"
Name Max Greenberg
Sex Male
Age 28 years
Birth Year (Estimated) 1850
Father's Name Nathan
Father's Sex Male
Mother's Name Amalia Lowenthal
Mother's Sex Female
Spouse's Name Debora Katz
Spouse's Sex Female
Spouse's Age 21 years
Spouse's Birth Year (Estimated) 1857
Spouse's Father's Name Aaron
Spouse's Father's Sex Male
Spouse's Mother's Name Sarah Schuff
Event Type Marriage
Event Date 10 November 1878
Event Place Manhattan, New York City, New York, United States
Source Details 109