Official Practice Exams 1 & 2 share the same question pool. The answer for your last question is YES. Taking Exam 1 four times is same as taking 1 & 2 two times each.
I wouldn't suggest taking Official Practice Exams more than 4 times. Even in your 3rd & 4th attempt you may get 20% to 30% repeat questions in each section. But that's okay. After 4 attempts, you will get most of the questions repeated, which might inflate your score much.
The difficulty level of questions, especially in quants, is higher in other mock exams like
GMAT club tests and
Experts Global. That might be the reason for lower scores in other mocks.
I strongly recommend you to purchase Official Practice Exams 3 to 6. Each purchased exam will give you two attempts so totally 12 mock exams, which reflects the real GMAT questions experience. No other mock exams can give you the same experience, though their higher difficulty levels are a good practice. If you are tight on budget, you can purchase Exams 3 & 4 for $60. This might even save you another GMAT attempt which is $275, if you utilize is efficiently.