The way I see it is like this:
1. All exercises prior to solo are completed to a consistently satisfactory standard. These should at least include stall recovery, PFLs, EFATOs
2. The student has flown recently with an instructor. How recent is a matter for the FTO.
3. The weather is within limits for student solo flight.
4. The FOB or company ops manual is followed to the word.
5. The student is properly briefed and is clear on what is expected to be completed during the flight.
6. The student is clear on what to do if things don't go to plan e.g. radio failure, orbiting, go-arounds etc.
7. All the above is written up in their student records prior to solo.
If all the above is completed and then things go wrong - I would suggest you can present to the AAIB or CAA or CFI the actions you took to ensure the student was correctly equipped to complete the flight.
I agree though - its a can of worms.....TJ