Okay, so a little tooling around and the solution is to "borrow" Light's code, not Fire's. When the damage formula is changed from:
"a.mat * (a.enhanceSkill?.().id == 169 ? 4 : a.enhanceSkill?.().id == 168 ? 3 : 2) - b.mdf * 2"
to:
"a.mat * (2 + (a.enhanceSkill && a.enhanceSkill() != undefined && a.enhanceSkill().id == 169 ? 2 : 0) + (a.enhanceSkill && a.enhanceSkill() != undefined && a.enhanceSkill().id == 168 ? 1 : 0)) - b.mdf * 2"
Splash I's damage (and Water Skin) works as intended. As an aside, trying to have a nested if:else statement (as in && a.enhanceSkill().id == 169 ? 2 : a.enhanceSkill().id == 168 ? 1 : 0) results in the error that returns Splash as 1 damage. Probably, RPGMV can't handle the logic and is returning a zero, but I'm not a coding or RPGMV expert so I couldn't tell you. There might be a more elegant solution, of course, but this at least works.