Thus you are subject to risk from BTC variations, without being able to hedge it.
Is the BTC/USD rate the one from MtGox ?
BTC/USD rate is irrelevant for your trading limits and whatnot.
Yep, but it'll affect your margin. If BTC/USD is up, your margin is increased, and vice-versa. Right ?
Accually no, if you want to go into details, margin is affected by EUR/USD rate, but its going in your favour anyway.
There seems to be no way around this until the BTC will be one of the 'official' Forex currencies. You can read about margin details here: http://btctrading.wordpress.com/2012/10/01/714/