Yeah, "Combo: 0" means if timed perfectly it connects on the last stun frame, "String: -1" means they have 1 frame to act (the frame the knee hits), etc. At the lower ranges you hit with the lower part of the knee when full jumping it, so even though they may have 2 frames to act they can't do anything about the knee since a jump would just have them get hit by the top of the knee instead. String: -1 is avoidable with a jump at the higher ranges though since you're hitting with the top of the knee in those cases.
U-throw advantage shows how many frames you can act while they're still stunned. When you don't need to move before jumping into the knee, you can see it starts true comboing (would register in training mode) at 78% which gives +18, since his jump is 4 startup and the knee comes out on 14.
Note that the advantage table applies to every character and not just Sheik, since the speed of his u-throw animation is unaffected by weight (d-throw however is which is why it sucks on heavies), and throw knockback/stun is unaffected by weight as well. +10 advantage allows u-air to combo (4 jump & hit on 6) as long as you don't need to dash into it and they are close enough. The start of the ranges is easier to combo off of since their stun time goes up by 1 while having the least KB.
Jiggs moves around while holding the crouch, and they can poke up and into range of some things during parts of the animation. The very start of the crouch is always at/near its lowest though.