5.

Q: Suppose you want to use timestampig for solving conflicts between a write
and a read, and locking to solve conflicts between writes. Under which
conditions does this seem like a better idea than just using locks?

A: In general, When read operations occur after an older transaction has written
an item, with locking the readers would block waiting for the item to be
released. With timestamping the readers could go on and read the item.

The presented scheme works if the writer does not want to write again an item it
already has written once, since the timestamp of the item has advanced beyond
its own, so it can advance and should be rolled back (or all the transaction
that have already read the item). This would cause the read transactions to be
rolled back as well, since they have read a value, that is no longer valid. I.e.
cascading rollbacks.

So, the scheme could be useful, if transactions only write the data items once 
and old transactions rarely want to make writes.