Full disclosure, the last time I studied chemistry was 20 years ago, and I was not a particularly good student, so take this with a heaping helping of NaCl.
It isn't the direct reaction of Drano + PVC that causes the issue. Rather, it's the heat given off from the reaction of the clog and the lye. Apparently it's significant enough to be an issue. I tried looking up how much heat might be released by the reaction, but I went crosseyed reading the formula, so someone else will have to do the math on that one.
Also, I know you said caustic material doesn't react with metal, but Google doubts you on that front, for whatever that's worth. In fact, zinc is specifically called out as a metal with which sodium hydroxide reacts pretty strongly, which is important because many water lines are steel galvanized with, you guessed it, zinc.
You could put Daniel Day-Lewis in that role and I think it would still be a terrible film. A good actor cannot rescue a story that inept. They might be able to wring some pathos from otherwise inept dialog, but it's not going to change the fundamental structural issues with the movie, and, as you allude to, it might strip it of what makes it entertaining in the first place.