Ok, I see that problem 2 has been resolved in cassandra https://issues.apache.org/jira/browse/CASSANDRA-1072Distributed counters will combine increase operations so everything will be consistent at the end.
Problem 1 (idempotency) is an open issue: https://issues.apache.org/jira/browse/CASSANDRA-4775
At least I know that people think about that, it is really important for my scenario so I will try to help resolving that issue.
I hope that this helps clarifying my question.
