Why didn't Hana just become Den-O? She's infinitely more competent than Ryotaro, so what was the purpose of drafting him into the position when there was already a perfectly capable singularity point available?
Because she's not a Singularity Point. She's a Junction Point, someone whose existence controls the flow of time itself. (And of course, because she's a girl).
But no one knew she was a Junction Point until near the end of the series.
Maybe she tried to become Den-o but couldn't and just didn't know why.
Alternatively, she could become Den-o, but she wasn't willing to let any Imagin possess her and was thus limited to using Plat Form.
The problem with that theory, though, is that she had Ryotaro become Den-O before either of them considered the idea of having Momo possess him.
Besides, she's much more tolerable once she becomes a little kid, and her attitude and fashion sense are much more befitting her appearance.
Ryotaro is a Singularity Point. It was stated time and time again throughout the entire series that Singularity Points have Ripple-Effect-Proof Memory, and it was shown that the memory-erasing effects of the Zeronos Cards don't work on him. Then, in the final arc, it's revealed that Ryotaro's memory of Airi's pregnancy was removed by a Zeronos Card. What the hell, Kobayashi?
Maybe he only became time-proof after it was brought to his attention?
Since his sister talked to him about it and explained that he needed to forget, I believe he can forget only if he is willing to do so.
So, only a Singularity Point can become Den-O, right? Okay, so explain to me how Momotaros and the other Imagin can all use a Den-O belt without possessing Ryotaro at various points in the series. It's been shown that these guys are just as susceptable to the time-change as anyone else (take, for example, Yuto being killed in the past and erased from history. Deneb only remembered ever being contracted to Ryotaro, and nobody else knew who Yuto was). So how can non-Singularities like the Imagin become Den-O?
In the movie Final Countdown (I believe anyway), Hana gives them what I believe are rider passes and they were able to transform with them without a host since they are in physical form and they could use them for transforming if they possess other hosts. (Sorry if that didn't make sense)
I get how they suited up; my question was why they were able to, given that they've been shown not to be Singularity Points.
It might be because of their link to Den-O that they're able to do so
You don't need to be a Singularity Point to be Den-O; just check the Decade story arc, where the Taros possesses Tsukasa, Natsumi, and Yuusuke and all three become Den-O at different points in the episode. (The evidence that they're not SPs is that they had absolutely no control while possessed.)
But they're dimension-hoppers; not exactly following the regular rules in the first place.
Plus, Tsukasa probably counts as a Singularity Point anyway.
My theory is that anyone can become Den-O, it's just that Singularity Points with their Ripple-Effect-Proof Memory that are the best to do so since they can remember what has been destroyed in the past and fix it. It was stated that since their memories are like this, the past will be restored because they remember it as it is.
Am I the only one bummed that Sieg never got his own version of Climax Jump? :(
Nope. I kind of wonder about that too.
Did Hana knew that Ari and Yuuto (not the past one) were her parents?
As demonstrated by the K-Touch, the sand that Imagins are made of is reactive: it can change its shape, form, and basically all of its physical properties by exposure to a person's "wishes." The glasses probably come from the "spray" of sand produced when someone undergoes possession; ditto for the changes in clothing.