It depends on what you're calling a cult.
I have met those I believe to be Adepts, who are quite unassuming people actually. The least likely folks to start an organisation like Eckankar. That being said some may get what they are seeking out of it. I am all for a religion which teaches techniques of Soul/Astral/Shamanic Travel as central to the practice. As opposed to mainstream sects which have no such options for exploration. However, when certain ridiculous dogmatic positions creep in, such as declaring yourself to be a singular prophet for one "true" current.. red flags begin to wave.
As for the organization itself I have read the autobiography of the current ECK master Sri Harold Klemp, which is very interesting and I recommend to anyone with an interest in such books. Not because he presents a view of reality I find particularly compelling, but because it contains a gripping narrative that is in equal measure an honest exploration of the visionary state, and an unconscious exploration of deep psychosis. He enters fugue states where the security guards at parks he is wandering around in are seen as Angelic Messengers, other ECK masters, etc. All while holding down a job as the on-staff Eckankar graphic design specialist.
Basically the way that he became the ECK Master was that Darwin Gross, the previous ECK Master, wanted out of the Guru game but he still liked the $$$, so he tried to retain financial and administrative control while making Klemp the new ECK Master. Probably seeing that he was a true believer and also that he was crazy enough to be easily manipulated. But Klemp eventually got wise to it and kicked him out.
I have gone to an Eckankar meeting. Not a small one but a larger one at a convention center so I could spy on these people and see what was up with them. My unkind impression from the combination of their style of dress and graphic design aesthetic is that they desperately wish they could be Scientologists. Klemp now communicates to them via a Skype-like video connection so he never has to leave Minneapolis.
I should admit I have a bias against this man, Klemp, as he cursed me and any other reader directly with the wrath of the SUGMAD if they should dare to be laughing at him in the process of reading his autobiography. Which I was, uproariously. At times deeply from the gut. It is true unconscious comedy genius, and it can't be denied that what makes it so funny is that the man is (or was at the time) crazy as ****. And if you can't have a sense of humor about that, so little in fact in that you have to make a point to CURSE PEOPLE in your autobiography, then I very much doubt your Mastery of ECK, HU, or any such cosmically-charged letter combination.
To answer your first question as to whether the ECK Masters prior to Paul Twitchell exist.. well I suppose they could just as much as any other spiritual being. Does that mean that Eckankar is the one true religion? I would say no. And also the fact that they, like Scientology, have so many warning pages up by ex-members is also a bit alarming.