It's not just the diffuser, and it's not just all the parts (front wings, barge boards, etc.) that attempt to seal and control the diffuser.
Williams need to go back and figure out why their technical processes (CFD, wind-tunnel, on-track instrumentation), and human processes (interpretation and reconciliation of all the technical info) were not reliably identifying problems that now seem to have been going on in-reality on-car on-track for some time (years?). Then they need to identify the technical problem. Then they need to identify technical solutions.
Possibly partial answers exist for middle or later issues already, but it's necessary to flesh out the processes in the earlier stages if you want to get anywhere. It's kind of like the first step of recovery from alcoholism: It starts by admitting you have a problem. Now that's a challenge for Paddy!-- He will either fail at this because he's a newcomer, or he will succeed at this because he's a newcomer.