Interestingly, Mr Bamberger himself appears to have been a lawyer.
Not necessarily. At the time, the honorific "Esq." was used to designate any gentleman. Its use only for attorneys is a modern development. Edit: the pencil notation on the cover suggests the cover came from an attorney's file handling a bankruptcy or reorganization. "In re Evarts (assignee) ... see letter enclosed."
Ira enjoyed women's hats?
Or men's. A milliner makes both.
As to the cancels, I will take a stab. The origin point (600 Broadway) is just below Houston St in today's SoHo and the destination (291) is near City Hall and what was the main Post Office at Park Row and Broadway. The cancel on the stamp looks like an ordinary duplex hammer that designates station A which was in SoHo and the arrival back stamp just says PO because it was the main office.