The point they are trying to address is to avoid IV catheterizations that have caused problems with some of the horrendous injectable drug protocols in the past including the horrendous OK attempt to use midazolam, a sedative; vecuronium bromide, a paralyzing drug, and potassium chloride, which stops the heart--only to find half those drugs painfully entered the tissues spaces from a bungled infiltrated catheter.
In theory, Nitrogen induced-anoxia would not need medical professionals, but it is also dangerous to those in the room should a leak develop. And a mask or hood for delivery would undoubtedly require some sedation and/or a secured head to ensure proper mask placement and maintenance.
Sedatives are notoriously variable in dose-related effects so drugs that also afford some restraint properties (e.g., ketamine, haloperidol). For a prisoner to be afforded their "last words" as a right of the condemned, heavy doses of diazepines, or opiates to assist with preventing struggle would be problematic.
Those who think this would be free of problems have never dealt with violent people, psych patients, or wildlife, for that matter. And the thought that this would be painless is naive' in real-world settings.
As one who is and always has been anti-death penalty, I would still hope that those making the decisions on this would be well informed by those who have the background and expertise. That said, those who would WANT to consult may well be the last ones society should WANT to do so. In many of these states, expediency that will minimally pass muster with the courts is their goal and a process more akin to compassionate euthanasia is decidedly not.