GMAT Simulations- Need help!

Hey,

From my experience its true.
You can take each test a few times (2-3) without getting the same questions.
Since it is an adaptive algorithm, just like in the actual test, The questions pool is much larger than the 37+41 questions that you get.
I believe that each one of the 2 official tests have a different questions pool.

About the second question, I can only help with "800Score" since I bought their full pack.
I must say I was disappointed with their quantitative and verbal questions, since they are not too bad but they are built in a different style comparing to the actual test, so in my opinion it is not too helpful. But you can take their free demo and decide for your self. I'm sure that all the sites that you have mentioned have a free demo, so take advantage of it.