Your understanding is 100% correct. The remaining questions are:
(1) How do we enter this BSL if the main application does not initiate this BSL -- due to any reason? The answer may be using one of otherwise unused pin, or a special power-up condition of the BSL TXD signal from the PC.
(2) How do we protect this BSL from being altered unintentionally by the main application? The answer may be partition the FRAM and use the MPU to provide some protection of this BSL.