For the slightly less biased answer

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There is some debate as to just what the rules mean on this.
One argument is that when the rules say that the number of powers you can cast is dependant on your mastery level, it means that the number of powers you can cast is
equal to your mastery level. This is not directly stated in the rules, however.
Another argument holds that since the rules say that you can keep casting until you run out of Warp Charges, and the actual dependancy relationship is never explained, you can cast as many powers as you have, provided you have sufficient Warp Charges. This argument requires you to largely ignore the line about casting being dependant on your Mastery Level.
The third argument is that the rule saying that casting is dependant on your Mastery Level is meaningless without some statement as to
how it is dependant, and so as the rules currently stand there is simply no way to determine how many powers a unit can cast.
As Kyutaru said, this was thoroughly hashed out here:
http://www.dakkadakka.com/dakkaforum/posts/list/599772.page
What it boils down to is that, until
GW FAQ it, you'll need to discuss it with your opponent.
For your second question, yes, if you don't roll enough dice to score a double 6, you won't suffer perils.
Automatically Appended Next Post: Never mind - just realised that this was the previous thread, dredged up again. Not really much point revisiting it again yet, as nothing has been fixed.