The GMATPrep tests are certainly better indicators of your score than unofficial tests. So you are in the range to get the score you are looking for.
You do not only have unofficial tests left. In fact you can take each of the GMATPrep tests twice and even three times. Certainly if you take each of them twice you will only get a few repeated questions. If you take them a third time you may see a few more. However, as long as you work through the problem as if you had not seen it before you will get a good practice.
I would not recommend taking the Kaplan tests that you have such a problem with this close to the examination. You do not need the psychological hit of another 400s score. And as I like to say, "the closer you get to the test date, the closer your practice should be to the test questions." In other words official questions are the best from here out.
Good luck -- you can certainly make it.