Steve.....
I have been designing for clients a long time. I ask for a deposit. When I receive the deposit, I begin to put together examples of website. Each one is a mock up. Meaning What they gave you must have been the very first mockup which with a few changes would be quite fine for your business. It's very likely the site is not even finished yet. It takes time to finish a site. I work one page at a time, until after a week, two, three, the site is finished. Of course it must be met by the clients approval. Then when the site is complete, they pay the balance that is due.

We adviced you to get a contract so if you did not, you will have no legal leg to stand on. The contract would have answered all your questions. How many pages and I entitled to, how many edits can I make to the initial layout, is SEO services involved, When the site would be completed, etc.

Your best bet now is to be NICE to them, stop acting like they are cheating you and find out "nicely" what it would take to finish this site.

Threatening legal action on them, is not going to endear you to them. I can only imagine how they are feeling on their end. They have submitted a site to you in good faith of being paid in full for their services. Now you are threatening not to pay. I realize I do not know all the details because you have been quite vague.

Let them do their job, they are the professionals, they obviously know what they are doing because the site got good reviews in the critique forum.

Calmly find out where you are in the process, what is left to do, and how much you owe them for a finished website.