I have these sentences, can you help? thanks

Hi I am new to this site, if i am doing this wrong please correct me but i need your opinion and was so happy when i came upon this site. English is not my strong suit but i try and reason as you will see below.

Just what would you do, Cheryl, if the situation were reversed?
(Am I correct to say remove the comma before Cheryl?)

The new machine, which I haven't even learned to operate yet, is out of order.
(I think it is correctly punctuated and yet i think you should remove the comma after machine)

Did the Novelist Ernest Hemingway once live here?
(I am leaning toward Correctly punctuated, but still i think i should add a comma after Hemingway)

Queen Mary wanted her death to be remembered, therefore, she staged a pageant.
(I am thinking change the comma after remembered to a semicolon or should I just remove the comma)

A first class employee is: dependable, loyal, and efficient.
(I would add a hyphen after first as you use a colon after an independent clause when a series follow right?