thanks guys, yes, this is a rather difficult one. 
@ianoc : thanks, i don't know if i fully understood your comment, but i still want to make use of counters (yes i'm aware of get and set style concurrency problems here) to have merges come around the non-idempotency of sums (counters apply vector clocks to achieve eventual consistency). @rubanm : i understand why you suggest that, but i don't fully grasp how to implement that - do you want to use a while loop (with timeout)? afaik it is not possible to perform lightweight ta on counter columns, even with cql