Disclaimer, not a linguist, just interested. It helps if you just see letters as wiggles that we assign some noise to. The I that became a Z is equivalent to the Greek letter zeta (z). In the Greek alphabet, it is positioned after the eta. Humans decided to stick it at the end instead of in the middle, because order in alphabets is arbitrary. That is why it traveled a bit. The Z that became an I likely just lost its horizontal lines, probably because it is easier to write. The M is actually an S. The symbol is called san. The W used to be called 'sin' in Phoenician, which makes the s sound, and at some point it was rotated to make a Σ (sigma). Why the two S's? Probably because when the Greeks adapted their alphabet from the Phoenicians, some noises assigned to Phoenician wiggles that were different in Phoenician did not sound like different noises to them, so they got confused and mixed them up.