1: You can take the test prep anywhere, with anyone. They cannot use the ISC2 logo, say they're associated, or claim accreditation without actually being ISC2-approved, but you're free to learn from anyone you'd like.
2: As previously stated, all the study material is readily-available. I liked both the Harris books and the CBK for different reasons. Whatever you buy, make sure you get the test simulator software with it. It really helps.
3: ISC2 wouldn't have much of a leg to stand on when it comes to test prep, unless the test prep company was doing something stupid like using the ISC2 logo, claiming affiliation, or using actual exam questions in test prep. You, as a customer of the offending party, should be exempt from any liability. However, I'd super-highly recommend sticking with an ISC2 affiliate. Why? They can use actual test questions during the training. They don't use current questions, but they do use past ones, which gives you a ton of insight into the nature of the questions you'll encounter on the exam. The seminars aren't cheap, but this is the kind of test you don't really want to take twice.
Good luck!
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You only say it's impossible because nobody's done it and lived.
