A: Some textures with different colors, (e.g. Terracota, TNT) when scaled to small size, can look not similar to their original size.

A: This can caused by reaching height limit, including blocks, that are not in the current game version or spawning blocks out of your max render distance.

A: You can reduce number of commands, if you optimize your image to include more simillar blocks.

A: Right now 1.13 version of WorldEdit is not stable and current .schematic file structure is not compatible with 1.13 blocks. You can read more about it here.