Mevrouw
Always use the date your are writing the letter as the date of the letter. UNLESS, you are writing the letter on a date before when you will send the letter. In this case, use the date that the letter will actually be sent.
Yes
The proper way to address a female boss in writing is to use Dear Miss and then the name. If she is married then use Ms. or Mrs. instead.
Most people Understand
Use the prefix of Mrs. if it is a fomal letter. If not, and you are familiar with them, use their first name.
What ever you want to use
When writing a letter, it is appropriate to use Mr.
Calligraphy is used for fancy writing for like a love letter or just a letter
Since Women's Liberation came into play during the 1960s some women do not want to be titled as Mrs. or Miss so they have shortened it for both married to single women to Ms. It is not that common now to use 'Ms' but a good idea to use it in a letter if one does not know if the person they are writing to is a Mrs. or a Miss.
Use Microsoft Word
You are writing a business letter, you should be more formal.
respected