[Effendi]

Let's try to answer to all the questions...

1) I think both German and French sim cards are activated at once when you buy them in the shops.

2) French prepaid cards are expensive, but billing is per second and MVNOs have usually cheaper rates

3) Usually it's cheaper among EU countries, about 1eur/min, anyway it's no sense to buy an European card for roaming...

4) It's the "old-style" way to make data calls calling an internet provider with your phone at 9,6Kbps... Most EU carriers offer at least GPRS to prepaid customers so you won't need CSD.

5) Voice calls always have the same rates with GSM or UMTS, it really doesn't change. Videocalls are usually more expensive, but I suppose you won't do any.
Data calls are expensive (not in Germany with some MVNOs) but there are sometimes some good offers.