Please avoid repeating the same question in different forums. Request you to post a question only once.

GMAT prep tests has only two tests that come with it. You could retake the test but the questions may be different from the earlier two to some extent, with remaining being a repeat.

To improve your score by more than a 100 points a this stage, you will need to very clinical in your approach. Identity the areas where you are weak, you will have to carefully review each test you have taken to list down the areas and question types you have got wrong. Work on each of them.

GMAT prep is the best of what is available today. KAPLAN generally gives you lesser scores. I have no idea on ARCO but I have heard they are easier than the actual ones. So I would suggets that you plan on the basis of your GMAT prep scores.

The score depends on the difficulty level at which you answered consistently well. It does not directly depend on no of questions you got right or wrong.

60-70 prep tests

I think you should focus on the OG in very qualitative terms. Identify the areas where you are weak and work on it. You cannot improve your score by just taking prep tests. Devise a plan that would work for you.

If you have got 650+ without any prep, in my opinion 720-730 looks like a target you can definitely achieve. I think you should include the other OGs in your prep. But you already too much to cover in 6 weeks I guess. So plan your time, devise a strategy, have check points through periodical tests. ...