Your grammar hurts my brain. When that happens, EX kills the source of the pain so be careful for future posts.
In the Bardock special, Bardock forseen Goku destroying Furiza, but Goku didn't, Trunks did... you get my point.
No I don't actually. O_o Bardock saw the future because of what happened to him on Kanassa. Goku didn't because he can't see the future. Trunks didn't see the future, Freeza's death happened 2-3 years before his birth.
Does Goku and/or Kurririn ever find out that Jackie Chun was actually Roshi?
Nope. Only Tenshinhan thanks to Tsuru-senin.
(This is for opinions) Don't you think it's kind of dumb about the SSJ legend only happening every thousand years, and not even all the elite Sayia-jins go SSJ, but later in DBZ kids who are like 7 who can't even fly are reaching this level? (Another version of this question is: Do you think Toryama should've allowed Trunks and Goten to become SSJ at such a young age?)
It's possible that they were playing "catch up" with their dads. Only Goten didn't know how to fly. Note that whatever sparked the legend was a solitary Super Saiyan amongst saiyans all with PLs under 18,000. When Goten and Trunks are born, Goku, Gohan and Vegeta are the three most powerful beings in existence, all of which can go at least SSJ.
How did Vegita learn to read power levels without a scouter on earth? How did he pick that technique up?
Through observation of the fighters. He's just a super-skilled fighter, and picked up the technique as a result of analyzing how they were fighting. Note that at that point elite strength fighters were just "blips on his psychic radar". He just knew that people were nearby, not exactly how strong they were or precisely where they were. He developped the skill more on Namek as time progressed.
How come it took Nappa so long to fight the Z Soldiers, it should've taken him less than a minute, right?
Yup, but Nappa's a fuck. That was his version of playing.