As to why the PM/PNF was not more assertive; after he told the PF to be more gentle with SS movements while PF was “stirring mayonnaise” (and PF had taken back control when PNF had started to correct the nose up condition), PNF concentrated on calling back the CDB. It seemed to me that the way CDB had handed control to PF before he left the cockpit may have implied that PF was not only PF but acting PIC too.
This would explain (not justify, but explain) both why PF felt he could seize control back from PNF, and why PNF then gave up trying to override PF and instead wanted the CDB back to assert his authority.
Admitted speculation, but it fits who did what.
(I tried to find the CVR transcript again but can’t – but I did read some of it that reached the public, which had that implication as i read it.)