Can fmDataGuard be used to provide transaction-level roll back (all or nothing)?

Yes. By setting a Transaction ID flag that gets included in your Audit Log, you can use the Roll Back feature to find these entries and roll each of them back, including Add, Change and Delete operations across multiple files, table and records. Note that unlike selecting Roll Back while working with ODBC/JDBC, your changes will be committed to your datasource as you go, but the Audit Log allows you to reverse the changes that were made after the fact. This will not prevent other users or processes from see the initial changes made in the period before you select to Roll Back.