Yes, I believe Paul Twitchell plagiarized the works that form the basis of his religious movement Eckankar. I also believe that no one in any religious movement would care in the slightest that he did. Religions aren't new and groundbreaking things, they're just someone with charisma writing down stuff that a lot of people already accept as common sense.
Given the belief system of Eckankar I believe it is easy to see that Paul Twitchell plagiarized the works to form the basis of his religion. I think this worked because followers probably were not familiar with the sources, making it easier for him to report his beliefs as original.
Twitchell has developed his religion, Echankar, from a sect of the Sikh religion. Very little of his belief system is original. Humans on this planet are typically plagiarizing one thing or another no matter how you look at it. Taking works or ideas that have already been had and presenting them as your own defines plagiarism. Since that is exactly what Twitchell has done, he has plagiarized.
Paul Twitchell did rely heavily on other works to form the basis of Eckankar, but that is not unique in religious movements. Many of the world's religions have evolved from other religions. Perhaps he could have better cited some of the basis, but no religion is formed in a vacuum.
No, I do not think that Paul plagiarized other works to form the basis of his new religious movement. He simply studied other religions and built upon them to form his new religion, which he and his followers believe to be superior to all of the other religions around today.