During his time, there was a lot of dissent, and alternative scriptures were being banned and buried, etc.
Constantine was confronted with a situation where Christianity was in better shape than what was left of the Empire, and figured he and the Empire would be better off working with 'em rather than ag'in 'em. Also, the Roman Pantheon started to lose credibility when the Roman Senate started to vote on deification for the recently deceased emperors and whatnot. By the time of Constantine, it was a laughingstock.
Yes. I have read Eileen Pagal The Gnostic Gospels. Constantine surely wanted first and foremost to control and contain this upstart religious movement, because he knew he couldn't supress it. BTW, I have read that Christianity was still a minority religion as the time the Emperor elevated it to official status, and as you point out, one with many contending factions. The Empire faced enough external threats at the time without being torn apart internally by religious feuds. Still, the official sanction of Christianity and the 50 Bibles came before, not after Constantine's conversion.
Secularists like to have a good time blaming religion for all the evils of history, but I believe it is the mixing of religion with politics (ie, the hijacking of religion by political forces) that is to blame for the worst things that have happened 'in the name of religion'. Just look at the Islamic theocracies of today!