Really sorry for the late reply!
1.”I’ll upgrade to 9S3P on each SPF. And yes, this configuration won’t exceed 450V even below -20°C.”
What do you mean by “I’ll upgrade to 9S3P on each SPF”,
2. “should I add external melting fuse for safety to cut off in rare case of internal FET break down ?
Just to prevent layout current overload in this failure mode.”
For off grid inverter, there are so many mosfets inside inverter, especially DC-DC circuit, you can try to add the melting fuse on AC output and observe if it will descrease the malfunction. What if it really works, please kindly let me know.