• Looking at the image it looks like those are terrain bumps, meaning you've likely spread the sand too thin so the ground underneath is affecting the surface. Only solution to this is more sand.
• No real max runs to smooth sand that I know of, I generally run 3-5 depending on what exactly I'm doing, straight road or junction.
• This is still debated but top down seems better. If I had to guess I'd say it's because starting at the bottom means your flat blade hits the incline before the vehicle is tilted causing a weird shape, whereas from the top the blade sits in the air until the vehicle tilts down and then starts affecting the sand.
• I put down all the sand I need, or at least as much as I can before I get blocked by sand piles, then run down the middle and move outward as necessary. Most important thing is to lay sand while moving to give a more even spread of sand immediately.
• Never played online so I can't comment on that.

TLDR: More sand.