You can dash over Ky's 2D, but its active frames out run your hopping frames. You can dash>OD and not worry about whether or not you made it through his active frames, but to do anything else to him you have to time the dash over the trailing end of his active frames.
f.S will whiff unless you catch Ky very early in 2D's startup, or very late in its recovery. Possibly only the first and last frames of Ky's 2D are hittable with f.S? You say he's using 2D while under pressure? At level 2, HS>MC,f.S is a true blockstring. You can frametrap him by delaying the f.S to catch his 2D, but if he's hitting you then you are either delaying the f.S too long, MCing too slow, or aren't looking out for the IB on your HS. But I still wouldn't use f.S, even as a frametrap. Ky has too many ways around it. f.S in this matchup is mostly a reactionary poke against Ky's 6HS.
Johnny's 6HS and 5K can be used to force a whiff. 6HS, of course, moves your hitbox back, and 5K you hit him with the leg that would of got you hit. Johnny's crouching block hitbox is wider than his crouching hitbox, so there is a distance where you can crouch and Ky's 2D will whiff, but if you down-back then you block it. Beating 2D, and the matchup in general, is mostly about spacing and some decent reactions.