If the Ventara Riders are kept in stasis until needed, how come they have Earth counterparts who are the same age they are?
This might be a case of a time lapse between dimensions occuring at some point. Kit mentions Ventara is supposed to be at least somewhat more technologically advanced than Earth. The fact that it seems to be implied that the massive structure that Xaviax uses for a base might have been the Riders base before they were Vented seems to confirm that at least some of their tech is way ahead of ours.
If the Earth Riders are exact DNA duplicates of the Ventaran Riders, why aren't the Ventaran Axe and Spear siblings just like the Cho brothers?
DNA doesn't determine your relationship with your brother, your intelligence, your dedication to family, or your inclination for vengeance.
Sorry, I wrote badly the question and it was misunderstood. I meant: Why the ventaran versions aren't siblings?
It's never stated that they aren't. The fact is that they don't share the same last name. They might be half-siblings, and we just ddidn't need to know it since it would be implied.
why didn't the big bad give the rider in the army the advent deck that gives BFG's if he was convincing him it's a new wepon
Because only the Sting deck would work on him. That's also why Xaviax couldn't use anyone he wanted.
Why do the Ventara Riders need to be put in stasis? If they did mentioned in the show, I forgot...
Eubolon was thought to be gone forever, thus new Kamen Riders can't be made.Say if a big alien threat happens in 100 years, they may need the Kamen Riders. Normally the old riders would have retired and new younger ones would have been made to replace them, but without Eubolon, the current Riders would have grown old and the riders are gone in a generation. So they're stretching out the ones they have as long as possible.
How come Kit had to contract with Dragredder, while all the other Earth Riders' Advent Decks were already contracted with their Advent Beasts? In fact, why would Kit's Advent Deck not be contracted with Dragredder at this point, considering that Adam has already contracted with Dragredder?
It's quite possible that Xaviax himself disengaged the contract as per Adam's agreement with him. Recall that all Adam wanted was to be out, and he did mention in one of the later episodes that Xaviax DOES deliver his end of the deal ("At least Xaviax gives you a fair deal"). This would've allowed Adam to spend time with his girlfriend uninterrupted.
On the same note, why was Kit having nightmares of fights between KR Dragon Knight and Onyx? He didn't have Onyx's Advent Deck at that point, nor was there any indication that Adam had any ties to Kamen Rider Onyx. I know it foreshadows later in the show when he becomes Onyx, but at the point where he's having the nightmares, there's no reason for it. And I know that later, one of the Ventaran Riders tells him that nightmares are the Advent Beasts trying to assert themselves, but for Kit to be having nightmares about Onyx, he would already have to have some connection to it, which he doesn't gain until after Eubulon retrieves him from the Advent Void and gives him the Onyx Advent Deck.
Maybe the nightmares is how they choose the 12 riders out of possibly hundreds of applicants.
This is actually explained in-universe. Kit's nightmares of becoming Onyx and going Ax-Crazy and attacking the other Riders are actually attempts by Dragredder to take over Kit's body. According to Len and Price, this is normal, and all the Riders have to deal with it, but eventually they fade away.
As for Dragredder, it's possible that because Adam just left instead of being vented, Dragredder decided to break the contract until it got a new user.
Actually, sting has a contract card and eventually has to contract with his manta ray(wait... why did I think it was manta and not sting?) advent beast as shown in one of his flashbacks when he was with maya before strike attacked them.
How come Len sensed an un-transformed Earth Incisor, but didn't know that JTC is Earth Strike until it was too late?
JTC has the wealth of experience of working under Xaviax. He can easily go under the shadows and will not make his intentions manifest until the ugly truth blows in his enemies' faces.