So at this point I'm thinking this: BS and NR aren't prototypes but instead are attempts at copying the build driver system and perhaps meant to be used by normal humans not exposed to nebula gas since I can't see the main bad guys taking the risk that they'd either die or end up a SMASH.

NR was exposed to the Pandora box light when it created the wall though, we already know the event altered the people there on a mental level so it is not impossible that it would also make them hazard level 3.0.

You are probably correct though, but unlike those who have become riders due to the gas, they likely have a limit to their strenght level, while the nebula gas riders are able to go past that level, knowing Kamen Rider in general though, that might have some very bad potential effects in the long run.

Here's my theory. Takumi did not have a Hazard level high enough to use the Build Suit, so he came up with some way to body swap and made plans to lure two promising candidates, Taro and Ryuga, so he could gas them up and find one with Hazard level 3. He lucked out with Taro, but when he tried to body swap, it didn't work quite right. Instead of a straight out swap, he got a mingling of sorts. Taro's body ended up with Takumi's intellect and much of his personality, but Taro's sense of decency, while Takumi's body ended up with the memories of both, the ambition and ruthlessness of Takumi, and the more carefree personality of Taro. The later is Blood Stalk. He's now trying to complete his works, but needs Sento to do it, since he no longer is intelligent enough to do all the work himself. He might even be fooling Faust into thinking he's still as smart as ever, which might explain the way he seems to turn on them for his own ends.

Definitely not the only one in the Yukari Taki Stan Club, lol.