Probably one of the biggest effects of Constantine moving east to what became known as Constantinople was that the Pope became important as a secular ruler; which was to have all kinds of unforeseen and unpleasant consequences for the Church.
Constantine is not considered a saint in the Catholic Church, only in the Orthodox Church.
You may read a full discussion of Constantine at the link below. To the best of my knowledge the Catholic Church, which is the Mystical Body of Christ made no concessions to Constantine, rather, Constantine's mother was a Catholic, and Constantine was a catechumen, converting on his deathbed. Constantine, also legalized Christianity, and stopped the persecution of the Christians.
Constantine is only considered a saint in the Orthodox Church and was not canonized or declared a saint in the Catholic Church.
Roman is an epithet first commonly used in England after the protestant revolt to describe the Catholic Church. It is rarely used by the Catholic Church. Constantine was Emperor of Rome who removed the penalty for being a Catholic, he, himself, converted before he died, but was not a Catholic for most of his life. He did not divide the Church.
He started the Roman Catholic Church.
In the Orthodox Church he is. Not in the Catholic Church, though his mother, Helena, is.
Constantine is not recognized as a saint in the Catholic Church so has no feast day. He is recognized as a saint in the Orthodox Church, however.
The Roman Catholic Church has always had its Capital in Rome.
Emperor Constantine did not found the Catholic church, Christ himself did through the 1st Pope St. Peter. Constantine converted the pagan Roman empire to Christianity in the 4th century after receiving heavenly signs prior to a battle which he won.
You are confused, first of all, there is no "independent Catholic grouping", there is just the world-wide Catholic Church under the successor of St. Peter. Our Blessed Lord has only established One Church, and, as it is His Body, It will remain ONE until the end of the world. Secondly, Constantine did not select anything, and did not "formalize" the Catholic Church. All Constantine did was to make Christianity legal. The center of the Catholic Church as always been in Rome since St. Peter went there, and died there. Constantine legalized Christianity, but did not become a Christian, himself, until on his death bed.
When the schism came to a close, near the time of the Council of Constantine, three popes where vying for the official position: Gregory XII- Roman Catholic Church Benedict XIII- French Catholic Church John XXIII- German Catholic Church
Constantine the Great is not considered as a saint in the Catholic religion. While he did make a death bed conversion to Christianity his life up to that point was anything but a saintly one. He is considered a saint in the Orthodox tradition. There are a few other Catholic saints named Constantine, however.