1) There are ways to avoid lvl 3-4 wild barrels, but usually learning this for just running boards KS is overkill. The little amount of time you spend on those screens usually only translates to 1-2 WBs max, sometimes more sometimes less. My advice to you is just restart it doesnt take all that long and get to 5-1 without dying. Million point players on the other hand spend 3x as much time on those boards as you do, which in effect means they see around 3x as many WBs. So its much more valuable to million point player than just someone running boards. I wouldn't worry about this until after KS.
2)Ok the easiest way to jump an x_x_x barrel combo is backjump, backjump, straight jump, in that exact order. You can do this one of 2 ways
a.Doing a standing backjump, ie standing still and waiting until the barrel almost touches mario then jump in the same direction as the barrel. when you land its probably best to run with the barrel, let the 2nd one nearly get to marios heels then jump in the direction of the barrel again. Once you land just straight jump the last barrel.
b. Do a running backjump, ie running the direction the barrel is going, waiting until the barrel is on marios heels and jump with the barrel, then just follow the rest of the steps.
3) on the cement level after you take bottom hammer you need to do these things in this order to achieve maximum safety potential.
a.Grab the hammer and as the conveyer is still moving to the left, run all the way to the left side of the screen, once you reach the edge stop holding left and left the screen push you left. If a fireball comes down the left ladder you turn right just enough to get the hammer to turn then either let him come to you (since all fireballs head left from ladders) or by this time the conveyor will be switching right and you just left the conveyor push you into the fireball.
b. When the conveyor switches right (THIS IS VERY IMPORTANT) you need to position yourself in such a way that the hammer is contact the left ladder, ie if a fireball comes down the left ladder he goes straight into your hammer. You are going to have to stutter step left with the hammer in your hand to keep your hammer even with the left ladder. If a fireballs comes down the left ladder you got him easily. If a fireball comes down the right ladder, you can use the conveyor (which is still moving right) to push you into that fireball for the smash.
these positionings on the cement stage will help you a lot . GL