I have replied below to your questions:
What are my chances of getting an offer after having passed the first round of interviewing for the German BCG Offices?
That could vary a lot by office and company needs in a particular moment, on average you can assume a ratio between 20% and 40%, assuming your second round is the final one.
How much should I prepare? (This is tuning on a high level, think 4 hours ("a little") as opposed to 8 hours ("a lot") a day?)
I would personally go all-in, whatever the level of preparation you have. If you do as much as you can and still don’t get the offer, at least you know you have given 100% on that. If you do less than that and don’t get the offer, you will always wonder if you could have done better. Since an MBB offer can be a life changing experience from a career point of view, I would not take the risk of ending up in the second scenario.
What all-in means in terms of hours is of course related to (i) the feasible time you have available and (ii) when you meet null or decreasing return on additional learning hours, which is something to be assessed on a person-by-person basis. I would also keep at least half of the day before the interview for a review-only of previous material/cases, in order not to stress too much with new material close to the interview.
Hope this helps,