Puzzle answers

Answers:
1. Driving all the way to Denver, Carol's car got great mileage.
(Carol's car didn't drive to Denver, Carol did.)
Driving all the way to Denver, Carol got great mileage in her Hummer.
Or:
Carol got great mileage on her drive to Denver.

2. To increase your chances of getting published, your manuscript should be error-free.
(Your manuscript can't do anything to help you get published, even if it had the will to do so.)
To increase your chances of getting published, make sure your manuscript is error-free.
Or:
Your manuscript should be error-free to increase your chances of getting published.

3. Repeat it again for everyone to hear.
Repeat it for everyone to hear. ("again" is superfluous)

4. Buy your special someone a diamond from Mo's Jewelry; they have the widest selection.
(Mo's is a business, an "it," not a "they")
Buy your special someone a diamond from Mo's Jewelry; it has the widest selection.