i hope somebody really knows the answers to your questions because i would also like to know.
i considered submitting designs to magazines but i won't do that until i have a nice healthy inventory. for one thing, if a magazine publishes your pattern, they will almost certainly require you to sign over all copyrights and it's no longer yours. for another, if it's a hit you'll want to be ready for the customers it draws in search of your other patterns.
the only way i can think of to "test" for originality is to spend a lot of time searching the internet and browsing through books and magazines to see what's already out there.
i'm sure somebody here will know more than i do about this. i can't wait to read. :-)