Shiro assumed there would be thirteen different rider this timeline so when he got someone for Odin he gave them that knowledge

That is my guess.

It's not like the Rider War started when Shinji became a Rider, it's conceivable enough that the other Riders just happened to die before the story started. I mean, the first Ryuki did.

It's possible that Verde and Femme could have killed each other prior to Shinji getting into the Rider War. Although I'm not so sure about Ryuga since it depends on Shinji's shadow.

But she isn't bald...

She only did 90 sit-ups, 90 pushups, 90 squats, and a 9km run every day. Unfortunately, She just needed 10% more to achieve baldness.

That's what I always assumed but then it raises even more questions. Like are they even the same people as they were in the movie + special? I suppose Itsuro could have always been there, but IIRC, part of the reason Miho participated in the Rider War was to exact revenge on Asakura, who wouldn't have been a Rider yet (though I suppose she'd still be involved to save her sister). And of course, Ryuga is suppose to be Dark Shinji, yet Shinji wouldn't have been a Rider yet either.

Kamen Rider Decade is a walking "unrevealed things" series by itself (the Decade summer movie revealed some origins, but gave us more "unrevealed things" to wonder about)

This.

Also, why does Wizard like donuts so much? Do they power his magic?

What made Gentaro from Fourze want to gel his hair up and look like a punk?

If Eiji from OOOs has his underwear on a stick, does that mean he's going commando?

Psychosis. He was eating one when his parents died so now he eats plain sugar donuts like it's a security blanket that reunites him with his dead parents.

Also keep in mind that he holds loose change in another pair, and he's visiting the cleaners very often.