You wrote:
“How certain are you ot the authenticity of this Epistle?”
Certain enough since Orthodox, Catholic and many Protestants all attest to its veracity. The anti-Catholics believe differently because it satisfies them to do so. If many Protestant scholars can believe it - and orthodox Protestants as that goes - why is it that anti-Catholics do not? It certainly isn’t because they have any real argument.
Do you believe Ignatius wrote three versions of his letters? The Short Recension, the Mid Recension, and the Long Recension? If you believe that it is fine with me. If you don't believe that you must explain where the "extra" versions came from. Further, you mut choose the one "authentic" letter.
From one of my favorite "anti-Catholic" sources.
The oldest collection of the writings of St. Ignatius known to have existed was that made use of by the historian Eusebius in the first half of the fourth century, but which unfortunately is no longer extant. It was made up of the seven letters written by Ignatius whilst on his way to Rome ; These letters were addressed to the Christians
of Ephesus (Pros Ephesious);
of Magnesia (Magnesieusin);
of Tralles (Trallianois);
of Rome (Pros Romaious);
of Philadelphia (Philadelpheusin);
of Smyrna (Smyrnaiois); and
to Polycarp (Pros Polykarpon).
We find these seven mentioned not only by Eusebius ("Hist. eccl.", III, xxxvi) but also by St. Jerome (De viris illust., c. xvi). Of later collections of Ignatian letters which have been preserved, the oldest is known as the "long recension". This collection, the author of which is unknown, dates from the latter part of the fourth century. It contains the seven genuine and six spurious letters, but even the genuine epistles were greatly interpolated to lend weight to the personal views of its author. For this reason they are incapable of bearing witness to the original form. The spurious letters in this recension are those that purport to be from Ignatius.
Catholic Encyclopedia - Ignatius Of Antioch
Obviously the author of the Catholic Encyclopedia did not have the confidence in the authenticity of any of the letters which you apparently do. I imagine he deserves the label "anti-Catholic" also.