Concerning costs. For option 2), the prepare message should piggyback 
the version identifiers of *each* data item that needs to be write-skew 
checked...which may lead to big messages, if you needed to test a lot of 
data items. But the ws-check is done only on the data items that are 
both read and written within the same xact. So I'd expect that normally 
just a few keys would need to be write-skew checked (at least this would 
be the case for the wide majority of DBMS/STM benchmarks I've been using 
so far).  Therefore I would not be too concerned with this issue.