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

Proceedure peformed

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.