Is it a good idea to omit HSE in favour of HSI?
HSI16 and HSI48 are both reliable.
HSI16 +/- 2% accuracy over the whole temperature interval is extremely good for an internal oscillator.
HSI48 is less accurate but still reliable.
SPI clock recovery is far less critical than USB because it has a dedicated clock pin.

For safety critical applications use an external hardware watchdog to control your CPU code execution.