no, not unless its a proper noun or adjective.
Yes, you should capitalize the letter after a comma in a sentence.
Yes, you should capitalize the first letter after a comma in a sentence.
Yes, you should capitalize "To Whom It May Concern" in a cover letter.
Yes you should.
If being used alone at the end of a letter then it should be capitalized. If it's following the word "yours" then it shouldn't be capitalized.
No, typically you do not capitalize after a slash unless it is a proper noun or the beginning of a sentence. It is more common to keep the letter following the slash in lowercase.
No. it's not necessary.
Only if it is the first word in a sentence.
Yes,it is a five letter word.
Post Script (PS) is a marker that the following in an afterthought. So the following sentence or paragraph should begin with a capital letter.
In "Sincerely yours," you should capitalize only the first letter of each word (Sincerely Yours).
You should capitalize the first letter of the sentence "Who is your favorite French artist?"