Here's where you started this discussion: "I cannot tell by looking at the W2 if the imputed income was included already."

The pertinent details you've given are:

"payroll taxes withheld on $4,500 of income = $2,000"

and...

"net check $2,500"

These details clearly show that the $300 is from takehome pay as a Post Tax deduction. So, the amount has not gotten any tax benefit treatment through payroll. As I pointed out, it's the same as if these people personally paid for the RDP coverage, such as, the RDP has an individual policy through Covered CA and not the employer of the other person.

"W2 has same taxable income box 1 and 16"

The CA wages is not changed from what you see on the W2. You keep asking about reducing the Wages. But what you really have is the condition of: If there is a CA tax benefit on entering the Premium Payment cost, this $300 is that amount. The $500 is not, because it already got favorable tax treatment by being paid pre-tax through payroll deduction. For instance, CA Medical Itemized Deduction would not include $500 but would include $300, if that is where you enter Health Care Premiums paid to exclude them from taxable income.

You keep asking for the wrong thing to do.