There is a true form of Christianity still today. Many people think Constantine did something great but in fact his decision open the door for paganism to enter Christianity creating the Universal (Catholic) church. When Christianity became accepted the truth was compromised for the sake of unity. You can see same thing happening today.
Yes once the Romans took over they got rid of all the communal and Gnostic parts of the religion and put the emphasis on death guilt and obedience.
The Nag Hammadi library is one of the only sources of pre-Roman Christianity and Gnosticism is the closest to real Christianity.
1) Did Constantine the Great toll the bell for the end of true followers of the Christ?
And bring in the rein of false Christians ?
Not as far as I know. If he did, I have seen no evidence suggesting that such was the case.
2) Is there a true form of Christianity that still exists ?
Seriously: probably not. I can't think of a single Christian sect (and that includes Jehovah's Witnesses) that has not been heavily influenced by non-Christian cultural elements and traditional practices and traditional teachings. Simple example: church buildings (or kingdom halls, if you prefer). Such were unknown to early Christians and are unattested in the Bible. Simple example: the position of hands during prayer (folded, or palms together, or fingers interlaced, or arms held out slightly from one's side) - again, all "positions" unattested in the Bible. Even our very language - used in religious worship - is chock-full of (etymologically) pagan religious elements.
However, there *are* many Christians who devoutly and sincerely strive to worship God in the manner taught by Jesus. These are true Christians - regardless of the "forms" of Christianity that exist.