Considering the following workflow.
Patient Checkin - admin form creation
Patient Invoice started - pre anaesethetic blood testing - automatic investigation generation.
Lab reports and docloader attaches to the investigation
Vet Invoices - uses template which adds another blood test (same one)
Vet then deletes the extra blood test but actually deletes the first test -
This causes a cascading delete which removes the investigation and associated reports.
I think that once a document is attached to an investigation that the investigation should be finalized and immutable. The associated invoice item can be removed but I would just remove the link rather than the investigation.