What if HMS Unicorn had accompanied Force Z?

Could an earlier HMS Unicorn have accompanied Force Z?

Possibly. An undelayed Unicorn could have been available in early 1941 and might have supplied Force Z with fighter support.

The counterfactual begins with the decision that delayed both Unicorn and the Illustrious-class program. Avoiding that delay could have put Unicorn into service by early 1941. She might then have been the spare aviation ship available to accompany Force Z. Her fighters would add an air-support capability the historical formation lacked. Their presence alone could not guarantee a different outcome.