> I don't think WAL can ever really have a durable transaction; it is essentially based on leaving a transaction open until it gets "check-pointed" or actually committed to the database file.

In general: WAL means you write the transaction to the WAL, fsync (in sqlite, this depends upon the sync mode], and then return it's done to the application. The transaction is then durable: even if the database crashes, the contents of the WAL will be applied to the database file.

Checkpointing later just lets you throw away that part of the WAL and not have to replay as much to the database file.