The rulebook says that an attorney can never, ever help a client to commit a crime. An attorney cannot act as a nominee for the client in furtherance of a crime – to shield the client. The attorney can't "take one for the team" thinking that because the attorney is acting in his capacity as an attorney, that's it's not a crime.
Attorneys don't have automatic immunity – and no, it's not just a rulebook violation. If Mr. Cohen wanted to shield Mr. Trump from knowing that campaign finance laws were being broken with this payment, then Mr. Cohen will be hard-pressed to avoid a criminal conviction, as he is as guilty as if Mr. Trump knew as Mr. Cohen is an aider, and abetter and a co-conspirator.
There is also conscious avoidance – Mr. Trump cannot close his eyes to the high probability that this payment is being made and campaign finance laws are being broken. Conscious avoidance allows the government to prove knowledge through the purposely closed eyes of the defendant.
The attorney can't have money pass through the attorney or the attorney's account – even the escrow account as a conduit or as a curtain. The attorney can't take care of a payment and not tell the client in order to protect the client from harm, without harming himself (the attorney).
If the client seeks to use the attorney's license to commit a crime, then any communications between the attorney and the client are no longer privileged. If the client needs the attorney to run money through the client, and the purpose of that is to circumvent the law, then those discussions are not privileged.