My publishing experience is limited to scientific writing for journals. But, I'm using the approach described below, in my first novel.
During the creation phase of a scientific manuscript (or presumably, a novel), it is best to use the formatting that is easiest for you.
Your job is to get through your creative process. It doesn't matter if you are single or double spaced, unless you are working on a typewriter (you say you are in Word) because you can reformat within word processors very easily. So, during the creation phase, use the formatting that you prefer. Use the font, and size you prefer. Use columns or not, as you like.
After the creation phase, you will want feedback.
Many readers prefer double spaced, so that they can write on a hard copy. 12 point is nice, because some of us don't see as well as others. Calibri and Arial are easier fonts to read according to most people (non-serif). You can use Times if you like, although it is serif, but be aware of the extra squiggles on the letters. Again, the font at this point does not need to be your submission font.
If you're sending an electronic copy around to readers, they can change the formatting in their word processor fairly easily. So, whether you format and print for them or send an electronic copy and let them format with a few quick keystrokes will be your discretion.
In scientific fields, it is common for manuscripts to be reviewed by co-authors electronically. We send files to one another and review them on our computers, in 'track changes' mode (you can find this under "review" on Word.) This nice feature keeps track of every change made by every coauthor, and when you, the original author, receive the feedback, you can see who made which suggestion and you can accept or reject it easily.
Your readers, of your writing, may also be able to review your manuscripts on a computer, and use track changes mode.
After you have a final manuscript to pitch to a publisher, you begin to worry about things like font, font size, word count, spacing, and so on - for their needs. At that point (unless you're on a typewriter in which case you need to know now) you go to their website - and each publisher (or journal) will be unique. You may need to format your manuscript distinctly (including changing word count!) for different publishers, until you find a taker.
The questions you are asking seem suitable for the final draft of your manuscript.
It is still useful to read through the final, formatted version (when you think you are ready to hit 'send') - because things look different in every iteration including the use of new fonts, the justification, and so on. (Aside: Printing a copy and reading a hard copy is highly recommended to catch errors that you simply don't see on an electronic copy.)
But, I have also seen authors try to match submission guidelines up front, as a means of reading the style that the publisher will receive, and this is not a bad idea. It is fine and fair to use the suggested formatting from day one. In that case, find your target publishers and see what they want.
Here is one example (for hard copy submissions, not electronic) from this website:
Manual Submission and Manuscript Format
Manuscripts submitted to Asimov's must be neatly typed, double-spaced
on one side of the sheet only, on bond paper (no erasable paper,
please). Any manuscript longer than 5 pages should be mailed to us
flat. Dot matrix printouts are acceptable only if they are easily
readable. Please do NOT send us submissions on disk. When using a word
processor, please do not justify the right margin. If sending a
printout, separate the sheets first. The manuscript should include the
title, your name and address, and the number of words in your story.
Enclose a cover letter if you like. All manuscripts must be
accompanied by a self-addressed stamped envelope (if manuscript is
over 5 pages, use a 9” x 12” envelope) carrying enough postage to
return the manuscript If you wish to save on postage, you may submit a
clear copy of your story along with a standard (#10) envelope, also
self-addressed and stamped. Mark your manuscript “DISPOSABLE,” and you
will receive our reply only. We do not suggest that you have us
dispose of your original typescript. If you live overseas or in
Canada, use International Reply Coupons for postage, along with a