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