I'd say the only thing a little bit off about Tatsumi Shinji in comparison to Kido Shinji was the quickness to desire revenge and accuse others of betrayal. However, since the whole point of the A.R. worlds was alternate universes, and the alternate interpretations of characters we already knew, it fit perfectly fine. Same with Haguro Len versus Akiyama Len. Heck, something similar like that happened briefly during the early episodes of Ryuki with Kido accusing Akiyama of using his Rider powers for "bad" purposes.
Also where did "200 years" come from? No time frame, other then a "year" since the betrayal and the Riders being formed and trained at an early age, 10-15 years tops, has been given for the Ventaran Riders timeline of events.
I'm more of the theory that "Cryo-Suspension" and "leaving the Earth behind" are references to space travel, likely the last ditch effort against Xaviax on a warship, spacefaring waypoint to his base, etc. After defeating Xaviax they were returning to "Cryo" before heading back to the surface of the planet and then everything just goes down the drain with Adam making his big betrayal and Xaviax stowing away on board and venting the Riders one by one. At any rate, Len's warnings to Kit about leaving the Earth might be the final step for the Riders or an emergency plan: remain in orbit to protect against any other threats/resume the conflict if Xaviax ever returns in some way.
Anyways, Onyx is more than likely Adam, though I foresee, if the story is anything like Episode Final's, Kit might briefly consider following a darker path with Xaviax as well. Possibly as a last ditch "ends justify the means" quandrary bit concerning his dad, even if only for a short "inner soliloquy/reflection" moment.