"What do you think about the 2018 retainer fees being deducted in 2019 when the lawsuit was settled?"

You would not deduct this in addition, because the prepaid retainer would have been used against the fees incurred in 2019. These would be like a downpayment and held until applied to the total owed by the client.

Or you described it incorrectly here: "some of those attorney costs were paid as a retainer in 2018"

You either pay Fees, or you prepay as a Retainer, to prove to the attorney there will be funds to apply to fees later. If the retainer was applied to anything in 2018, those were 2018 costs.