Do NOT, repeat NOT, use the widow's first name. You address the envelope just as though the husband were still alive. Example: Mrs. Robert Smith
While that is the formal way address, in today's society, many women prefer to be addressed with their own first name. Neither is considered incorrect.
Yes, it is proper to address a widow by her last married name.
Use the prefix of Mrs. if it is a fomal letter. If not, and you are familiar with them, use their first name.
what is the proper way to address the mayor
The proper way to address the conductor of a symphony orchestra is 'Maestro.'
The proper way to address an envelope for a Ph.D. is "Dr. [Full Name]" followed by their address.
the proper way to address the Queen is by "your majesty" or Ma'm, pronounced like "ham'
No, a widow generally remains as "Mrs".
It is always a sensitive situation when you need to address a widow. In order to show her respect you can address her as Ms.
"Your honor"
usted. or ustedes.
Gentlemen
The proper way to address a Doctor of Osteopathic Medicine (DO) (not Doctor of Osteopathy) is Doctor (Dr.) since they are medical physicians.