fall, go around
隧 centers on downward movement or a path that goes around: falling, or a route that winds or circles. The two given meanings are somewhat distinct, but both involve motion along a course.
隧 combines ⻖ (mound, hill) with 遂, which likely contributes the sound. The character is associated with a path or tunnel through a hill, and by extension falling or going around. The exact historical development is uncertain.
The left side ⻖ is a hill, and the right side 遂 suggests completing a path. Imagine a tunnel through a hill that goes around obstacles, but if you're not careful, you might fall into a hidden pit along the way.
For スイ, picture a swinging vine that goes around a hill and then suddenly falls: swing -> スイ, and the vine's path matches the kanji's meaning.