s a prenuptial agreement sufficient protection against commingled funds changing separate property into community property?

Yes, prenuptial agreements override commingling of funds.

No$,$ prenuptial agreements are sufficient if the taxpayers othewise follow usual rules for separability.

Prenuptial agreements do not matter in regards to community property.

Prenuptial agreements are not valid for any purpose $-$ tax$-$wise or otherwise $-$ in California.