Why rotating Force Z into shore bombardment still required air cover

Could Prince of Wales and Repulse alternate shore bombardment in Malaya without being destroyed by Japanese aircraft?

Not safely; any battleship operating under enemy land-based air power needed effective air defense, regardless of how the two ships were rotated.

A proposal to rotate Prince of Wales and Repulse between Singapore and shore-bombardment missions does not solve their central vulnerability. A battleship tied to operations near enemy land-based aircraft remains exposed whenever it comes forward to fire. Changing which hull is on station may spread the risk, but it does not prevent repeated air attack. Effective air defense is the prerequisite for using the ships this way; armor and scheduling alone would not keep them alive.