Hi Spartan,
Congratulations on seeking some heavy stuff.
When I started out in the early 90s, I hit the library and went through the UFO and spirituality section. The plus side was that these were published books, and were selected to be stocked in the library. So they were already filtered a bit, and each book presented its case from start to finish. That gave me a foundation.
The internet, on the other hand, has so much variety and at times loony-ness that it can be overwhelming knowing where to start and how to sort it out. So I would recommend a bit of both, books to ground and flesh in the details, internet to supplement and introduce.
Reason I say this is because the more material you chew through, the easier it becomes to tell the potential accuracy of a source. It's like being a connoisseur of some food or drink, with experience comes better discernment of the subtleties.
If you can explain to another what a source is saying, and why you agree or disagree with it, then you have sufficiently understood it to make that call. Otherwise it would be mere bias. Biased skepticism and naive gullibility are the two things to watch out for...
You brought up two interesting examples, the Phoenix Journals and the Cassiopaean Transcripts. If you had no background in fringe stuff and came upon both, you'd be taken aback by the equal strangeness between them.
With experience, you may see that one better correlates with your own research, and the other has more warning flags signalling fabrication or outright deception.
I believe that by being familiar with methods of deception and manipulation, you can more easily cut out the rotten parts and take the good stuff. Do that to both sources you mentioned... what do you like and dislike about each? If there is potential for deception, what in each source might indicate this? If there is potential for sincerity and accuracy, what in each source might show that instead?
For instance, you asked if there is any evidence that the Cassiopaean transcripts is by negative beings. I sense that there are portions of the transcripts that have an odd tone not fitting to the rest of the material, and that these are laced with more corruption than the rest. Overall, I don't sense that the Cassiopaeans are deceptive. Or rather, if they are telling lies, then I myself was born with these lies within me and have experienced them as practical principles in my life, because a majority of the material resonates with me and correlates with my own insights and experiences.
There's lots of stuff out there with a deceptive bent, but if you examine as many as you can then patterns appear. For instance, material by negative beings tends to have a fundamental tone of glib insincerity, authoritative pushiness, constant appeals to ego, rationalizations for all things that might arouse suspicions about the source, appeals to doom and survival, and empty complexity trying to pass off as profundity. Negative sources are also big on using prophecies, paranormal demonstrations, and extreme synchronicity to overwhelmingly impress those they are trying to ensnare. I know this from observing the same pattern among sources that inevitably reveal themselves to be deceptive.
Good material tends to have these qualities instead: supports what you yourself have experienced, observed, or gained through insight / explains things better and more completely than anything you have previously come across / is not riddled with concepts and suggestions you can easily disprove through counter-example / does not waste your time with endless trivia or superfluous language / makes sense and feeds your need to know, yet compels you to learn more for yourself / helps you attain new insights, which are really rememberings of what you already know / presents principles and ideas that can be tested through experience and are verified later / preaches either a service-to-self philosophy or makes one vulnerable to such beings / appeals to emotional stability and lucidity instead of overwhelming negativity or blissful positivity.
Do some research on NESARA and Dove, and you'll get a better perspective on the source behind the Phoenix Journals. Rather than dissect it for you, I'll just state my opinion that it all has the same vibe, uses the same tactics, invokes similar names from other odd channelings like Ashtar, Sananda, Hatonn, Germain, Gabriel, etc...
Comparing two sources is a good exercise... if two things contradict, then a) could the answer be a mix of both, b) might one need to be chosen over the other, c) can a new perspective show that neither contradicts the other, or d) are they both false?
Ultimately, it's your call on what appeals to you. As long as you don't betray your intuition, experience, insight, and reasoning, you won't get permanently off track.
Acquiring fringe knowledge is like digging for diamonds in a mine field.