"Next, go straight along the narrow path."
I picked the boxes in the following order and Duo said it was correct: つぎ、ますぐに... So i assume つぎ doesn't necessarily require a particle to be used correctly. Since both つぎ and ますぐ can be followed by に (one as a time particle, the other as a direction particle) but that Japanese people tend to avoid using the same particle twice in a single sentence (that's more of a feeling than a rule I guess) maybe both positions for に are acceptable and you just skip the other?
Because 狭い (せまい) is an -い adjective. As a rule, -い adjectives do not require a -な suffix when used before a noun, just their basic -い suffix. There are two Important exceptions: the adjectives for big (大きい or 大きな) and small (小さい or 小さな) are considered to have BOTH an -い form AND a -な form, and it is considered more correct to use the -な form when placing these adjectives before a noun. In most other uses, you use the -い form with these adjectives.