The scrolling in Sega's version is rougher than coleco's zaxxon.
Actually, it's the exact same, because it's tile-based scrolling, and tile-based scrolling always works in increments of 8 pixels on the ColecoVision (and the SG-1000). If it seems "smoother" on the ColecoVision version, it's because the scrolling is done slightly faster. If you look at top-down games like Bump 'n Jump or Spy Hunter, when your car is going at full speed, the scrolling seems smoother than when the car moves at a lower speed, but that's only because your eyes and brain have time to analyze the differences between the screen-refresh frames. The faster the scrolling, the blurrier it becomes, and your brain interprets the blurriness as "smooth scrolling". But it's always scrolling done in increments of 8 pixels, no matter the scrolling speed.
Did coleco outsource any programming to japanese companies?
I would tend to say no in general, but that's actually an interesting question. If you look at certain games that were released near the end of the commercial life of the ColecoVision, like Antarctic Adventure and Monkey Academy, they seem to be MSX ports, and looking at the MSX library, there are several MSX titles that seem like they were ported from the ColecoVision to the MSX (Alcazar, Rock 'n Bolt, Sewer Sam, Aquattack, to name a few) because they look exactly the same as the corresponding ColecoVision versions. So I'm thinking that, in that 1983/84 period, an exchange of game software was subtly beginning to form between Japan and America. Who did the ports of games like Antarctic Adventure and Monkey Academy to the ColecoVision, if they are indeed MSX ports, that is the real question.