Bao,
1. You are right, you have to wait for 90 days if you fail an exam. Here is the exact text (Pg 7 of the PRM candidate guidebook): “Please note that you may only take the same exam once every ninety days. If you repeat the exam before ninety days have passed, your second score will not count and you will be charged the full fee for both exams.”
2. NO. Let me say it again – NO – Riskprep cannot be your only source of study for the PRM exam. The exam is not trivial, and just practicing from this website (or any other website) is not enough. You have to go through the handbook. I do not know how deep your major in finance went into risk management, but generally I would think these are not as mathematical as the PRM exam, nor do they focus on risk. Try the free sample questions on RiskPrep, and maybe you will get some idea of where you stand.
I would also suggest you look at and read the following links:
http://www.riskprep.com/faq#5
http://www.riskprep.com/blog/think-you-are-ready-for-the-exam-.html
My best wishes,
Mukul Pareek