What happens when subcontractor has been paid in full for the job-- yet the same subcontractor never sent a 20 day preliminary notice and has not finished the job that was contractually agreed upon?
Preliminary notice is not relevant to this situation, as preliminary notice affects the subcontractor's collection rights and these rights should not come in to play if he has been paid in full.
The subcontractor should not have collected payment in full before completing the work. Generally it makes sense to demand that the subcontractor immediately return to complete its work without delay. If the subcontractor fails to do so, you may have remedies including legal action on the contract, a claim on the subcontractor's license bond, and/or a CSLB complaint.