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1. It is not possible (normally) to register with another persons address. Commune requires a rental agreement with your name on it. Also, as a part of the process, is the police verification when police will visit the address you provided for verification and check if you live there, etc.
2. Yes, it will be a risk. They cannot process your payment without a national number which you can obtain only by registration in commune.
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You need a fixed address and the police will visit there too